Musannaf Ibn Abi Shayba 21:417

Waki’ narrated to us, from Mis’ar, from Sa’id ibn Abi Burdah, from his father, who said: I heard Ibn ‘Umar saying: "My Lord, for that which You have bestowed upon me, I will never be a supporter of the criminals." When he prayed, he said: "I have not performed a prayer except that I hope it will be an expiation for what is before it." He meant that he said it while he was bowing.