Musannaf Ibn Abi Shayba 3:1037

Abu Mu'awiyah narrated to us, from Abd al-Rahman ibn Ishaq, from al-Nu'man ibn Sa'd, from Ali, who said: When he entered the mosque, he would say: "O Allah, forgive my sins and open for me the doors of Your mercy." And when he exited, he would say: "O Allah, forgive my sins and open for me the doors of Your bounty."