771. Sunan Ibn Majah Chapter: One Who Builds A Mosque For The Sake Of Allah; Book 4, Hadith 37

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