Musannaf Ibn Abi Shayba 3:1035

Ibn ‘Ulayyah and Abu Mu’awiyah narrated to us, from Layth, from ‘Abdullah ibn al-Hasan, from his mother, from Fatimah, daughter of the Messenger of Allah (may Allah bless him and grant him peace), who said: When the Messenger of Allah (may Allah bless him and grant him peace) would enter the mosque, he would say: "In the name of Allah, and peace be upon the Messenger of Allah. O Allah, forgive my sins and open for me the doors of Your mercy." And when he would exit, he would say: "In the name of Allah, and peace be upon the Messenger of Allah. O Allah, forgive my sins and open for me the doors of Your bounty."