Musnad Ahmad 28:144

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to us, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr: That the Messenger of Allah, may Allah bless him and grant him peace, when he would lie down for sleep, would say: "In Your name, my Lord, I have placed my side, so forgive my sin for me."