Musnad Ahmad 436:8

Waki’ narrated to us, Sufyan narrated to us, from Suhayl ibn Abi Salih, from ‘Ata’ ibn Yazid al-Laythi, from Tamim al-Dari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The religion is sincerity, the religion is sincerity," three times. They said: "To whom, O Messenger of Allah?" He said: "To Allah, and to His Book, and to His Messenger, and to the leaders of the Muslims, and to their common folk."