Musnad Ahmad — Hadith 24009/5

Hajjaj narrated to us, Sharik narrated to us, from Abu Ishaq, from Farwah ibn Nawfal, from Al-Harith ibn Jablah, who said: I said, "O Messenger of Allah, teach me something to say when I go to bed." He said: "When you take your bed at night, recite 'Say: O disbelievers' (__QUR_N__), for it is a declaration of innocence from shirk (associating partners with Allah)."