Musnad Abu Ya'la al-Mawsili 99:1

'Abd al-Wahid ibn Ghiyath narrated to us, 'Abd al-'Aziz ibn Muslim narrated to us, from Abu Ishaq, from Farwah ibn Nawfal, who said: I came to the Messenger of Allah (ﷺ). He said to me: "What brought you?" I said: "To teach me words [to say] when I go to my bed." He said: "Recite 'Say, O disbelievers' (Surah al-Kafirun), for it is a disavowal of polytheism."