Musnad Ahmad 436:2

Yahya ibn Sa'id narrated to us, from Sufyan, who said: Suhayl ibn Abi Salih narrated to me, from 'Ata' ibn Yazid, from Tamim al-Dari, from the Prophet, peace and blessings of Allah be upon him, who said: "The religion is sincerity. The religion is sincerity." It was said: "To whom?" He said: "To Allah, and to His Messenger, and to His Book, and to the leaders of the Muslims, and to their common folk."