Musnad Ahmad 30:242

Sufyan narrated to us, Ibn ‘Ajlan narrated to us, and it was read to Sufyan, from Sa’id, from Abu Hurayrah, who used to say—and Sufyan said: It is like this—meaning the Prophet (may Allah bless him and grant him peace): When he would lay down his side, he would say: "In Your name, my Lord, I have laid down my side. So 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."