Musnad Ahmad 1136:4

'Abd al-Razzaq narrated to us, Sufyan informed us, from Abu Ishaq, from Farwah al-Ashja'i, who attributed it to the Prophet, may Allah bless him and grant him peace, that he said to a man: "Recite when you go to sleep: 'Say, O disbelievers,' for it is an acquittal from polytheism."