Musannaf Ibn Abi Shayba 21:393

Muhammad ibn Bishr narrated to us, Mis'ar narrated to us, from al-Rukayn, from his father, from Umar, that he used to say: "O Allah, I seek Your forgiveness for my sin, and I seek Your guidance for the right course of my affair, and I repent to You, so accept my repentance, for indeed You are my Lord."