Musnad Ahmad 1123:1

Hajjaj narrated to us, Shareek narrated to us, from Abu Ishaq, from Farwah ibn Nawfal, from al-Harith ibn Jabalah, who said: I said: "O Messenger of Allah, teach me something that I may say when I go to my bed." He said: "When you take to your bed at night, recite: 'Say, O disbelievers' (Qul ya ayyuha al-kafirun), for it is a declaration of immunity from polytheism."