The Prophet (ṣallallāhu ʿalayhi wa-sallam) was an individual whose heart was most serene, and yet he was tested with a number of trials, with grief and sadness and yet he was pleased with the decree of his Lord. The believer should take the Messenger (ṣallallāhu ʿalayhi wa-sallam) as an example, remaining patient when bad befalls him and being grateful when blessed; for indeed, the affair of the believer – all of it is good. The Prophet (ṣallallāhu ʿalayhi wa-sallam) was persecuted and forcefully removed from his home in Makkah, and he suffered many difficulties; he was accused of being crazy, a sorcerer and a fortune teller, and yet none of that bothered him in the least because he had a heart which was at peace with the decree of Allāh.

Allāh (subḥānahu wa-taʿāla) says,

فَمَن يُرِدِ ٱللَّهُ أَن يَهۡدِيَهُ ۥ يَشۡرَحۡ صَدۡرَهُ ۥ لِلۡإِسۡلَـٰمِۖ وَمَن يُرِدۡ أَن يُضِلَّهُ ۥ يَجۡعَلۡ صَدۡرَهُ ۥ ضَيِّقًا حَرَجً۬ا ڪَأَنَّمَا يَصَّعَّدُ فِى ٱلسَّمَآءِۚ ڪَذَٲلِكَ يَجۡعَلُ ٱللَّهُ ٱلرِّجۡسَ عَلَى ٱلَّذِينَ لَا يُؤۡمِنُونَ
And whomsoever Allâh wills to guide, He opens his breast to Isām, and whomsoever He wills to send astray, He makes his breast closed and constricted as if he is climbing up to the sky. Thus Allāh puts the wrath on those who believe not.
[Sūrah al-Anʿām, 6: 125]

So the means to a tranquil heart is through strength of īmān, and if Allāh wants good for a person, He guides him to īmān. If Allāh pushes the servant towards the light of īmān, then this is a sign that Allāh wants good for the servant and it is a sign of the opening of the chest of the servant [towards the dīn]. The people of materialism may have the best clothing, and live in the best of places; however, it is of no benefit to them, and the reality is not as they depict it, and that is because they have lost the most important of affairs, and that is the affair of īmān in Allāh; that which if one has it, the heart is filled with calmness, serenity and peace. The believer, on the other hand, can be tried and restricted in terms of wealth, and his health and yet, due to his obedience to Allāh, he experiences a sweetness of faith and reliance which cannot be described, seen or heard; rather it is spiritual. The one who turns away from Allāh, making his sole focus the life of this world, then Allāh says concerning this individual,

ۖ وَمَن يُرِدۡ أَن يُضِلَّهُ ۥ يَجۡعَلۡ صَدۡرَهُ ۥ ضَيِّقًا حَرَجً۬ا ڪَأَنَّمَا يَصَّعَّدُ فِى ٱلسَّمَآءِۚ
And whomsoever He wills to send astray, He makes his breast closed and constricted, as if he is climbing up to the sky.
[Sūrah al-Anʿām, 6: 125]

And this is because he has missed that which causes the heart to be upon calmness and serenity, which can only be obtained through the sweetness of īmān. The means to achieving this calmness and serenity of the heart is mentioned by the doctor of the hearts, the Messenger Muḥammad (ṣallallāhu ʿalayhi wa-sallam). The first is the issue of tawḥīd, and it is the right of Allāh upon his servant, that He be worshipped alone and that none be associated with him:

وَلَقَدۡ أُوحِىَ إِلَيۡكَ وَإِلَى ٱلَّذِينَ مِن قَبۡلِكَ لَٮِٕنۡ أَشۡرَكۡتَ لَيَحۡبَطَنَّ عَمَلُكَ وَلَتَكُونَنَّ مِنَ ٱلۡخَـٰسِرِينَ
And indeed it has been revealed to you [O Muḥammad], as it was to those [Allāh’s Messengers] before you: “If you join others in worship with Allāh, [then] surely [all] your deeds will be in vain, and you will certainly be among the losers.”
[Sūrah al-Zumar, 39: 65]

Allāh (subḥānahu wa-taʿāla) also says,

فَٱعۡلَمۡ أَنَّهُ ۥ لَآ إِلَـٰهَ إِلَّا ٱللَّهُ وَٱسۡتَغۡفِرۡ لِذَنۢبِكَ
So know [O Muḥammad] that Lā ilāha illā Allāh [none has the right to be worshipped but Allāh], and ask forgiveness for your sin.
[Sūrah Muḥammad, 47: 19]

And also,

وَلَقَدۡ بَعَثۡنَا فِى ڪُلِّ أُمَّةٍ۬ رَّسُولاً أَنِ ٱعۡبُدُواْ ٱللَّهَ وَٱجۡتَنِبُواْ ٱلطَّـٰغُوتَۖ
And verily, We have sent among every Ummah [community, nation] a Messenger [proclaiming]: “Worship Allāh [Alone], and avoid [or keep away from] Ṭāghūt [all false deities, etc. i.e. do not worship Ṭāghūt besides Allāh].”
[Sūrah al-Nahl, 16: 36]

This message of singling Allāh alone in all acts of worship was taught by all of the Messengers and the Prophets (ʿalayhim al-Salām). Having ikhlās for Allāh and worshipping Him alone without calling on others besides Him, or slaughtering for other than Him, or seeking intercession; these principles must be understood and followed. It is obligatory that reliance, tawakkul is for Allāh alone. It is narrated by Ibn ʿAbbās (raḍiAllāhu ʿanhuma): [the statement] “Allāh is sufficient for us and He Is the Best Disposer of affairs,” was said by Ibrāhīm when he was thrown into the Fire; and it was said by Muḥammad when hypocrites said, “A great army is gathering against you, therefore, fear them,” but it only increased their faith and they said: “Allāh is sufficient for us, and He is the Best Disposer [of affairs, for us]” [Ṣaḥīḥ al-Bukhārī, Book of Faith, No. 4234].

Thus seeking aid from Allāh is a matter of tawḥīd, and submitting or swearing by other than Allāh is not correct, and this is something which the mushrikūn do. The believer will not fear anyone except Allāh. So this matter of shirk and tawḥīd is of the utmost importance and is the distinction between truth and falsehood; īmān and kufr. It should be learnt, understood and implemented, and it is through this actualization of tawḥīḍ which leads to the opening of the breasts to īmān, resulting in peace and tranquillity of the heart. Just as we should single Allāh alone out in worship, then similarly, we single the Prophet Muḥammad (ṣallallāhu ʿalayhi wa-sallam) out in ittibāʿ, following and adhering to his Sunnah and staying away from the doubtful and innovated affairs. Allāh (subḥānahu wa-taʿāla) says,

فَلَا وَرَبِّكَ لَا يُؤۡمِنُونَ حَتَّىٰ يُحَكِّمُوكَ فِيمَا شَجَرَ بَيۡنَهُمۡ ثُمَّ لَا يَجِدُواْ فِىٓ أَنفُسِہِمۡ حَرَجً۬ا مِّمَّا قَضَيۡتَ وَيُسَلِّمُواْ تَسۡلِيمً۬ا
But no, by your Lord, they can have no Faith, until they make you [O Muḥammad] judge in all disputes between them, and find in themselves no resistance against your decisions, and accept [them] with full submission.
[Sūrah al-Nisá, 4: 65]

Success and the opening of the chest and tranquility of the heart can only be found in the adhering to the Sunnah of the Messenger (ṣallallāhu ʿalayhi wa-sallam), and this was understood by the Companions (raḍiAllāhu ʿanhum ajmaʿīn) who knew with certainty that the one who follows the Messenger upon clarity, then Allāh will never allow him to be lost, or leave him in a state of difficulty so long as he is following the Messenger; however, that is not to say that he will not be tested with fitan, as Allāh (subḥānahu wa-taʿāla) mentions in His Noble Book,

الم ۞ أَحَسِبَ النَّاسُ أَن يُتْرَكُوا أَن يَقُولُوا آمَنَّا وَهُمْ لَا يُفْتَنُونَ ۞ وَلَقَدْ فَتَنَّا الَّذِينَ مِن قَبْلِهِمْ ۖ فَلَيَعْلَمَنَّ اللَّهُ الَّذِينَ صَدَقُوا وَلَيَعْلَمَنَّ الْكَاذِبِينَ
Alif-Lām-Mīm [These letters are one of the miracles of the Qurʾān, and none but Allāh (Alone) knows their meanings.] Do people think that they will be left alone because they say: “We believe,” and will not be tested. And We indeed tested those who were before them. And Allāh will certainly make [it] known [the truth of] those who are true, and will certainly make [it] known [the falsehood of] those who are liars, [although Allāh knows all that before putting them to test].
[Sūrah al-ʿAnkabūt, 29:1-3]

The believer will be tested but it should be understood that the good end is for those who follow the Qurʾān and the Sunnah of the Messenger (ṣallallāhu ʿalayhi wa-sallam). The Companions knew and understood that in following the Messenger there was ease and serenity, and it was due to this that they were eager to obey and carry out his orders. Those who are heedless and neglectful, then they are in a state of loss and they do not benefit from this blessing at all, they are as Allāh describes describes them in the Qurʾān,

قُلْ هَلْ نُنَبِّئُكُم بِالْأَخْسَرِينَ أَعْمَالًا ۞الَّذِينَ ضَلَّ سَعْيُهُمْ فِي الْحَيَاةِ الدُّنْيَا وَهُمْ يَحْسَبُونَ أَنَّهُمْ يُحْسِنُونَ صُنْعًا
Say (O Muḥammad): “Shall We tell you the greatest losers in respect of (their) deeds? Those whose efforts have been wasted in this life while they thought that they were acquiring good by their deeds!
[Sūrah al-Kahf, 18:103-104]

The third affair which is a means to attaining peace and serenity of the heart and an opening of the chest is the remembrance of Allāh. It befits the individual to understand the Sunnah of the Messenger (ṣallallāhu ʿalayhi wa-sallam) and the importance he gave to the remembrance of Allāh, so much so that the Mother of the Believers, ʿĀʾishah (raḍiAllāhu ʿanhā) mentioned that the Messenger of Allāh would remember Allāh in all of his conditions and tasks, and this is the most beneficial of medicines. Allāh says in His Noble Book,

ٱلَّذِينَ ءَامَنُواْ وَتَطۡمَٮِٕنُّ قُلُوبُهُم بِذِكۡرِ ٱللَّهِۗ أَلَا بِذِڪۡرِ ٱللَّهِ تَطۡمَٮِٕنُّ ٱلۡقُلُوبُ
Those who believed (in the Oneness of Allāh – Islāmic Monotheism), and whose hearts find rest in the remembrance of Allāh, Verily, in the remembrance of Allāh do hearts find rest.
[Sūrah al- Raʿd, 13:28]

Abū Musa Al-Ashʿarī (raḍi Allāhu ʿanhu) reported: The Prophet (ṣallallāhu ʿalayhi wa-sallam) said, “The similitude of one who remembers his Rabb and one who does not remember Him, is like that of the living and the dead” [al-Bukhārī and Muslim].

Ibn al-Qayyim (raḥimahullāh) in al-Wābil al-Sayyib mentions that Shayṭān looks into the hearts of the people, and when he finds them heedless whispers to them, and when he sees that they are upon the remembrance of Allāh, then he retreats. Ibn Taymiyyah (raḥimahullāh) mentions that dhikr to the heart is as water is to a fish, so look to what happens when a fish is taken from the water.

Also, from the means to attaining tranquility in the heart is being content with the decree of Allāh and His Will, and to have firm belief of this in the heart; so when one is going through difficulty, he is patient and that is good for him, and if he has been blessed then he is grateful and that is also good for him; the affair of the believer, all of it is good. As for the one who does not believe and is displeased with the decree of Allāh, then he is in a state of anger when afflicted with difficulties, and is arrogant when bestowed with blessings and he is not from those who are grateful.

Staying away from sins is also from the means to calmness and serenity of the heart, and this is important because sins are from those matters which can destroy an individual and takes one in the direction of other than that which is correct. It is important to understand that sins should not be considered a light matter, and the believer should seek to correct and rectify himself. Ways in which this can be achieved is by surrounding oneself with good companions, and accompanying those who are righteous and will remind you to fear Allāh, and who will correct you when you err; this type of companion will be a benefit for you and keep you away from that which is evil. However, if you surround yourself with companions who are upon corruption, then you will follow them in that corruption.