Musnad Abu Ya'la al-Mawsili 163:16

Al-Hasan ibn 'Arafah narrated to us, Muhammad ibn Khazim narrated to us, from Layth ibn Abi Sulaym, from 'Abdullah ibn al-Hasan, from Fatimah, daughter of the Messenger of Allah, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) 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."