Musnad Ahmad 30:2482

Yahya narrated to us, from Yazid ibn Kaysan, who said: Abu Hazim narrated to me, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said to his uncle: "Say: 'There is no god but Allah,' I will testify for you by it on the Day of Resurrection." He said: "Were it not that Quraysh would reproach me, saying that he was only driven to it by fear of death, I would have gladdened your eye with it." Then Allah, the Almighty and Majestic, revealed: "Indeed, you do not guide whom you like" [Al-Qasas: 56].