Musnad Ahmad 1136:2

Abu Ahmad narrated to us, Israel narrated to us, from Abu Ishaq, from Farwah ibn Nawfal, from his father—who was a foster father to Umm Salamah—who said: I came to the Prophet, peace and blessings of Allah be upon him, and he said: "What brings you?" He said: I came so that you might teach me something to say when I go to sleep. He said: "Recite {Say, 'O disbelievers'} when you go to sleep, for it is a disavowal of polytheism."