Musnad Abu Ya'la al-Mawsili 167:18

Abu Khaythama narrated to us, Ismail bin Ibrahim narrated to us, from Layth bin Abi Sulaym, from Abdullah bin Hasan bin Hasan, from his mother Fatima bint Husayn, from her grandmother Fatima bint 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) entered the mosque, he would invoke blessings upon Muhammad, then say: "O Allah, forgive my sin and open for me the doors of Your mercy." And when he exited, he would invoke blessings upon Muhammad, then say: "O Allah, forgive my sins and open for me the doors of Your mercy."