Musnad Ahmad 1061:10

Abu Amir narrated to us, 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, who said: I heard Abu Humayd and Abu Usayd saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you enters the mosque, let him say: 'O Allah, open for me the doors of Your mercy.' And when he exits, let him say: 'O Allah, I ask You of Your bounty.'"