Musnad Ahmad 30:836

Safwan narrated to us, Ibn ‘Ajlan informed us, from al-Qa‘qa‘, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The religion is sincerity," three times. He said: It was said: "O Messenger of Allah, for whom?" He said: "For Allah, and for His Book, and for His Messenger, and for the leaders of the Muslims."