Sunan an-Nasa'i 4198
سنن النسائي#4198SahihIt was narrated that Tamim Ad-Dari said:"The Messenger of Allah said: 'Religion is sincerity (An-Nasihah).' 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 39:50