Musnad Ahmad 1136:5

Yahya ibn Adam narrated to us, Sufyan narrated to us, from Abu Ishaq, from Farwah ibn Nawfal al-Ashja’i: That the Messenger of Allah (peace and blessings of Allah be upon him) said to a man: "Recite when you go to sleep: {Say, 'O disbelievers'}, for it is an acquittal from polytheism."