Musnad Abu Ya'la al-Mawsili 110:45

Abu Bakr ibn Abi Shaybah narrated to us, Zayd ibn al-Hubab narrated to us, Muhammad ibn Muslim narrated to us, from ‘Amr ibn Dinar, from Ibn ‘Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The religion is sincerity." They said: "To whom, O Messenger of Allah?" He said: "To the Book of Allah, to His Prophet, and to the leaders of the Muslims."