Musannaf Ibn Abi Shayba 21:116

Ibn Fudayl narrated to us, from Abd al-Rahman ibn Ishaq, from Muharib ibn Dithar, from Ibn Umar, who said: "When one of you prostrates, let him say: 'O Lord, I have wronged my soul, so forgive me.'" Muharib said: "For it is the closest one is to Allah, the Mighty and Majestic."