When any of you enters the mosque, let him say: 'Allahumma iftah li abwāba rahmatik' (O Allah, open for me the gates of Your mercy). And when he leaves, let him say: 'Allahumma inni as’aluka min fadlik' (O Allah, I ask You of Your bounty)

The Prophet (may Allah's peace and blessings be upon him) directed his Ummah to the supplication to be said upon entering the mosque: "O Allah, open for me the gates of Your mercy." He asks Allah Almighty to provide him with the means to attain His mercy. And when he wants to leave, he should say: "O Allah, I ask You of Your bounty." He asks Allah for His bounty and more of His benevolence in providing lawful sustenance.