Sunan Ibn Majah, Chapter: The Chapters on the Mosques and the Congregations

It was narrated that Fatimah the daughter of the Messenger of Allah (saw) said: "Whenever the Messenger of Allah (saw) entered the masjid (mosque) he would say: 'Bismillah, was-salamu ala Rasulullah, Allahummagh-firli dhunubi waftah li abwaba rahmatika. (In the Name of Allah, and peace be upon the Messenger of Allah. O Allah, forgive my sins and open to me the gates of Your mercy).' When he left he would say: 'Bismillah, was-salamu ala Rasulullah, Allahummagh-firli dhunubu waftah li abwaba fadlika. (In the Name of Allah, peace be upon the Messenger of Allah. O Allah, forgive my sins and open to me the gates of Your bounty).'" (Daif)