Musnad Ahmad 30:2461

Yahya narrated to us, from Ubaydullah, who said: Sa’id narrated to me, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you goes to his bed, let him shake out his bed with the inner part of his lower garment, and let him lie on his right side. Then let him say: 'In Your name, my Lord, I have placed my side, and by You I lift it. O Allah, if You withhold my soul, then have mercy upon it, and if You send it back, then protect it with that which You protect Your righteous servants.'"