Musnad Ahmad 1136:1

Hashim ibn al-Qasim narrated to us, Zuhayr narrated to us, Abu Ishaq narrated to us, from Farwah ibn Nawfal, from his father: That the Messenger of Allah (peace and blessings of Allah be upon him) said to him: "Do you have any interest in a stepdaughter of ours, so that you may take care of her?" He said: I think he meant Zaynab. Then he came, and the Prophet (peace and blessings of Allah be upon him) asked him about her, saying: "What did the girl do?" He said: I left her with her mother. He said: "Then what brought you?" He said: I came so that you might teach me something I can say when I go to sleep. He said: "Recite 'Say, O disbelievers' (Qul ya ayyuha al-kafirun), then sleep at its conclusion, for it is an acquittal from shirk (polytheism)."