Musnad Ahmad 266:9

Abu Amir narrated to us, he said: Sulayman ibn Bilal narrated to us, from Rabi'ah ibn Abi Abd al-Rahman, from Abd al-Malik ibn Sa'id ibn Suwayd al-Ansari, he said: I heard Abu Humayd and Abu Usayd saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you enters the mosque, let him say: 'O Allah, open for us the doors of Your mercy.' And when he exits, let him say: 'O Allah, I ask You of Your bounty.'"