ʿĀʾishah (رضي الله عنه) said:

I asked Allah’s Messenger (صلى الله عليه وسلم) about the verse:

وَالَّذِينَ يُؤْتُونَ مَا آتَوا وَّقُلُوبُهُمْ وَجِلَةٌ أَنَّهُمْ إِلَىٰ رَبِّهِمْ رَاجِعُونَ

“And those who strive in performing good deeds, with their hearts full of fear (lest their deeds are not accepted by Allāh)…”
(Al-Muʾminūn, 23:60)

Are they those who drink intoxicants and steal? He (صلى الله عليه وسلم) responded, “No, O’ daughter of (Abū Bakr) al-Ṣiddīq! Rather, they are those who fast, pray and give in charity, fearing that it will not be accepted from them.

أُولَٰئِكَ يُسَارِعُونَ فِي الْخَيْرَاتِ وَهُمْ لَهَا سَابِقُونَ

‘It is these who hasten to good deeds, and they are foremost therein.’”
(Al-Muʾminūn, 23:61).¹

Al-ʿAllāmah Muḥammad Nāṣir al-Dīn al-Albānī comments on the ḥadīth saying:

The secret behind the fear of the Believers that their acts of worship may not be accepted by Allāh does not emanate from a fear that He will not give them their rewards in full, for this is in contradiction to His promise to them in the likes of His saying:

فَأَمَّا الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحَاتِ فَيُوَفِّيهِمْ أُجُورَهُمْ

“As for those who believe and work righteous deeds, He will give their [due] rewards…”
(Al-Nisāʾ, 4:173)

In fact, He will multiply their rewards, as He said:

لِيُوَفِّيَهُمْ أُجُورَهُمْ وَيَزِيدَهُم مِّن فَضْلِهِ

“That He may give them in full their rewards, and give them [even] more out of His Grace.”
(Fāṭir, 35:30)

Surely Allāh does not fail in His promise, as He informs in His book.

However, the secret behind the Believers’ fear is that Allāh’s acceptance of an act of worship is contingent upon it being performed in the manner He—the Exalted—has commanded and ordained, and they are unable to pre-ascertain that they have fulfilled it in the manner He wants. Rather, they believe that they have fallen short in that regard. Therefore, they fear that their actions may be unaccepted.

So, let the Believer take heed and reflect upon this matter. Perhaps they may then increase in their diligence to perfect their acts of worship, performing them in the manner Allāh has commanded. However, this can only be achieved by way of sincerity and following of the Prophet’s guidance (صلى الله عليه وسلم) therein. This is the meaning of Allāh’s saying:

فَمَن كَانَ يَرْجُو لِقَاءَ رَبِّهِ فَلْيَعْمَلْ عَمَلًا صَالِحًا وَلَا يُشْرِكْ بِعِبَادَةِ رَبِّهِ أَحَدًا

“So whoever hopes for the Meeting with his Lord, let them work righteousness and associate none as a partner in the worship of his Lord.”²
(Al-Kahf, 18:110)