Musannaf Ibn Abi Shayba 17:1226

Al-Fadl ibn Dukayn narrated to us, he said: 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: "I came, O Messenger of Allah, for you to teach me something [to say] when I go to sleep." He said: "When you take to your bed, then recite 'Say, O disbelievers' (Qul ya ayyuha al-kafirun), then sleep upon its conclusion, for it is an acquittal from polytheism."