Musannaf Ibn Abi Shayba 21:184

Al-Fadl ibn Dukayn narrated to us, Zuhayr narrated to us, from Abu Ishaq, from Farwah ibn Nawfal, from his father, that the Messenger of Allah (may Allah bless him and grant him peace) said to him: "What brought you?" He said: "O Messenger of Allah, teach me something I may say when I go to sleep." He said: "When you take to your bed, recite 'Say, O disbelievers,' then sleep upon its conclusion, for it is an acquittal from polytheism."