Musnad Ahmad 436:6

Sufyan ibn 'Uyaynah narrated to us, from Suhayl ibn Abi Salih, from 'Ata' ibn Yazid al-Laythi, from Tamim al-Dari, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the religion is sincerity. Indeed, the religion is sincerity. Indeed, the religion is sincerity." They said: "To whom, O Messenger of Allah?" He said: "To Allah, and to His Book, and to His Prophet, and to the leaders of the believers, and to their common folk."