Musannaf Ibn Abi Shayba 21:148

Abu Bakr narrated to us, he said: Ubaydah ibn Humayd narrated to us, from al-Rukayn ibn al-Rabi’, from his father, who said: When Umar would finish his prayer, he would 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. O Allah, You are my Lord, so make my desire toward You, and make my sufficiency in my chest, and bless me in what You have provided for me, and accept from me, for indeed You are my Lord."