Musnad Ahmad 1154:4

Isma'il ibn Ibrahim narrated to us, he said: Layth—meaning Ibn Abi Sulaym—narrated to us, from 'Abdullah ibn Hasan, from his mother, Fatimah bint Husayn, from her grandmother, 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 invoke blessings upon Muhammad and grant him peace, and say: "O Allah, forgive my sins and open for me the doors of Your mercy." And when he would exit, he would invoke blessings upon Muhammad and grant him peace, then say: "O Allah, forgive my sins and open for me the doors of Your bounty." Isma'il said: I met 'Abdullah ibn Hasan and asked him about this hadith, and he said: When he would enter, he would say: "My Lord, open for me the door of Your mercy," and when he would exit, he would say: "My Lord, open for me the door of Your bounty."