Musnad Ahmad 30:820

Yazid narrated to us, Abdullah ibn Umar informed us, from al-Maqburi, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "When one of you goes to his bed, let him shake it off with the inside of his lower garment, for he does not know what occurred after him. And when he lays his side down, let him say: 'In Your name, O Allah, I have laid my side down, and by You I lift it. O Allah, if You withhold my soul, then forgive it, and if You send it back, then protect it with that which You protect Your righteous servants.'"