Narrated by Sufyān ibn ʿAbdullāh al-Thaqafī who said: “I said to the Prophet [may the peace and blessings of Allāh be upon him]: ‘Give me a statement I could not receive from anyone after you.’ He said: ‘Say: I believe in Allāh, then be steadfast.’” Narrated by Muslim.

Explanation:

Shaykh ʿAbd Al-Raḥmān ibn Nāṣir al-Saʿdī [d. 1376 AH] said:

This man requested from the Prophet [may the peace and blessings of Allāh be upon him] a statement that is comprehensive in benefit and goodness, that would provide for him an avenue to true success. So the Prophet [ﷺ] ordered him to firstly have belief in Allāh. This belief is comprehensive of that which is obligatory from the articles of īmān and its principles and what follows on from it, including the actions of the hearts as well as physical and mental wholehearted obedience and submission to Allāh. Then, to be continually steadfast in this state of being until death. It is the very enactment of the saying of the Most High:

إِنَّ الَّذِينَ قَالُوا رَبُّنَا اللَّهُ ثُمَّ اسْتَقَامُوا تَتَنَزَّلُ عَلَيْهِمُ الْمَلَائِكَةُ أَلَّا تَخَافُوا وَلَا تَحْزَنُوا وَأَبْشِرُوا بِالْجَنَّةِ الَّتِي كُنتُمْ تُوعَدُونَ ‎﴿٣٠﴾‏

‘Indeed, those who have said, “Our Lord is Allāh” and then remained on a right course – the angels will descend upon them, [saying], “Do not fear and do not grieve but receive good tidings of Paradise, which you were promised.’
[Fuṣṣilat, 41:30]

Thus, possession of true belief and steadfastness is directly correlated with safety from evil in all its forms, and the attainment of Paradise and all beloved things.

Moreover, there is a great plethora of authentic passages from the the Book [of Allāh] and the Sunnah that highlight the comprehensiveness of belief in it encompassing the contents of the hearts itself in terms of creed, but also its actions such as sincerely encouraging goodness, dissuasion from evil, desiring [the enacting of] goodness, and harbouring hatred for evil as espoused by the actions of the limbs. Such [stipulations of belief] could never be achieved without steadfastness.

Source: Bahjah Qulūb Al-Abrār: 23
Translated by: Riyāḍ Al-Kanadī