Musnad Abu Ya'la al-Mawsili 228:1

Narrated to us Mansur ibn Abi Muzahim, narrated to us Isma'il ibn 'Ayyash, from Suhayl ibn Abi Salih, from his father, from 'Ata' ibn Yazid, from Tamim al-Dari, that the Prophet (peace be upon him) said: "The religion is sincerity, the religion is sincerity, the religion is sincerity." They said: "To whom, O Messenger of Allah?" He said: "To Allah, and to His Messenger, and to His Book, and to the leaders of the Muslims, and to their common folk."