Hadith: Abu Ruqayyah Tamīm ibn Aws ad-Dāri (may Allah be pleased with him) reported that the Prophet (ﷺ) said:||"Religion is sincerity." We said: "To whom?" He said: "To Allah, to His Book, to His Messenger, to the Muslim leaders and their common folk

عَنْ أَبِي رُقَيَّةَ تَمِيمِ بْنِ أَوْسٍ الدَّارِيِّ رَضِيَ اللَّهُ عَنْهُ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ:
«الدِّينُ النَّصِيحَةُ» قُلْنَا: لِمَنْ؟ قَالَ: «لِلهِ وَلِكِتَابِهِ وَلِرَسُولِهِ وَلِأَئِمَّةِ الْمُسْلِمِينَ وَعَامَّتِهِمْ».
[صحيح] - [رواه مسلم] - [الأربعون النووية: 7]

Abu Ruqayyah Tamīm ibn Aws ad-Dāri (may Allah be pleased with him) reported that the Prophet (ﷺ) said:
"Religion is sincerity." We said: "To whom?" He said: "To Allah, to His Book, to His Messenger, to the Muslim leaders and their common folk."
[Authentic hadith] - [Narrated by Muslim] - [الأربعون النووية - 7]

Explanation

The Prophet (ﷺ) informed that the religion rests upon sincerity and truthfulness, to be observed as Allah ordained, in a perfect manner without shortcomings or deception. It was said to the Prophet (ﷺ): To whom should the sincerity be? He said: First: Sincerity should be to Allah Almighty: by performing deeds sincerely for His sake, not associating partners with Him, believing in His lordship, divinity, names, and attributes, revering His command, and calling people to believe in Him. Second: Sincerity should be to His Book, namely the Noble Qur’an: by believing that it is His speech and the last book revealed by Him and that it abrogated all the legislations before it—and by revering it, reciting it correctly, acting upon its definite verses and accepting its ambiguous ones, defending it against the misinterpretation of those who seek to distort it, taking lessons from its admonitions, spreading its knowledge, and calling people to it. Third: Sincerity should be to His Messenger Muhammad (ﷺ): by believing that he is the last Messenger, believing him in all that he came with, observing his commands and prohibitions, worshiping Allah only according to what he came with, venerating and revering him, spreading his call, disseminating his Shariah, and negating the false accusations from him. Fourth: Sincerity should be to the leaders of the Muslims: by helping them in what is right, not disputing with them over power, and hearing and obeying them in what is pleasing to Allah. Fifth: Sincerity should be to the Muslims: by showing kindness to them, calling them to Allah, averting harm from them, wishing good for them, and cooperating with them in righteousness and piety.

Benefits from the Hadith

1. The Hadīth commands us to be sincere to everyone.
2. Sincerity occupies great status in the religion.
3. The religion comprises beliefs, words, and deeds.
4. An aspect of sincerity is to rid oneself of deception toward the one to whom sincerity is addressed and to wish good for him.
5. The Hadīth highlights the Prophet's good approach to teaching, as he would briefly mention something and then explain it in detail.
6. First things first. The Prophet (may Allah's peace and blessings be upon him) began with sincerity to Allah, then to His Book, then to His Messenger (may Allah's peace and blessings be upon him), then to the leaders of the Muslims, and then to the common Muslims.