It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) 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."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا الرَّبِيعُ بْنُ سُلَيْمَانَ، قَالَ حَدَّثَنَا شُعَيْبُ بْنُ اللَّيْثِ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ عَجْلاَنَ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنِ الْقَعْقَاعِ بْنِ حَكِيمٍ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ " إِنَّ الدِّينَ النَّصِيحَةُ إِنَّ الدِّينَ النَّصِيحَةُ إِنَّ الدِّينَ النَّصِيحَةُ " . قَالُوا لِمَنْ يَا رَسُولَ اللَّهِ قَالَ " لِلَّهِ وَلِكِتَابِهِ وَلِرَسُولِهِ وَلأَئِمَّةِ الْمُسْلِمِينَ وَعَامَّتِهِمْ " .
sunnah.com