Musannaf Ibn Abi Shayba 3:1036

Waki’ narrated to us, from ‘Abdullah bin Sa’id, from ‘Amr bin Abi ‘Amr al-Madini, from al-Muttalib bin ‘Abdullah bin Hantab, that the Prophet, may Allah bless him and grant him peace, used to say when he entered the mosque: "O Allah, open for me the doors of Your mercy, and facilitate for me the doors of Your provision."