It was narrated that Abu Humaid As-Sa’idi said: “The Messenger of Allah (peace be upon him) said: ‘When anyone of you enters the mosque, let him send peace upon the prophet, then let him say: “Allahummaftah li abwaba rahmatika (O Allah, open to me the gates of Your mercy).” And when he leaves, let him say: “Allahumma inni as’aluka min fadlika. (O Allah, I ask of you from Your bounty).”
Sunan Ibn Majah, Book 4, Hadith 38