It is narrated from Hazrat Abu Humaid (may Allah be pleased with him) and Abu Usaid (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: When any one of you enters the mosque, he should say, "Allahumma iftah li abwaba rahmatik" and when he leaves, he should say, "Allahumma inni as'aluka min fadlik."