Hadith: Religion is sincerity

عن تميم الداري رضي الله عنه أن النبي صلى الله عليه وسلم قال:
«الدِّينُ النَّصِيحَةُ» قُلْنَا: لِمَنْ؟ قَالَ: «لِلهِ وَلِكِتَابِهِ وَلِرَسُولِهِ وَلِأَئِمَّةِ الْمُسْلِمِينَ وَعَامَّتِهِمْ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 55]

Tamīm ad-Dāri (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) 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] - [Sahih Muslim - 55]

Explanation

The Prophet (may Allah's peace and blessings be upon him) 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 (may Allah's peace and blessings be upon him): To whom should 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, venerating 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to His Book, namely the noble Qur’an: by believing that it is His speech and the last book revealed by Him and that it superseded all the legislations before it - and by revering it, reciting it correctly, acting upon its definite verses and accepting its ambiguous ones, defending it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, heeding its lessons and admonitions, spreading its knowledge, and calling people to it.
Third: Sincerity should be to His Messenger Muhammad (may Allah's peace and blessings be upon him): 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 accusations leveled against him.
Fourth: Sincerity should be to the leaders of the Muslims: by helping them in what is right, not fighting 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.