On the authority of Tameem ibn Aus ad-Daree (may Allah be pleased with him):
The Prophet (peace and blessings of Allah be upon him) said, “The deen (religion) is naseehah (advice, sincerity).” We said “To whom?” He (peace and blessings of Allah be upon him) said: “To Allah, His Book, His Messenger, and to the leaders of the Muslims and their common folk.”
Related by Muslim.
(Imam Nawawi’s Forty Hadith, Hadith no. 7)
In another hadith, it was narrated from Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said:
“Religion is sincerity, religion is sincerity (Al-Nasihah), religion is sincerity.” They said: “To whom, O Messenger of Allah?” He said: “To Allah, to His Book, to His Messenger, to the imams of the Muslims and to their common folk.”
Graded Sahih by Darussalam
(Sunan an-Nasa’i 4199)