Musnad Abu Ya'la al-Mawsili 167:19

Isma'il said: So I met 'Abdullah ibn Hasan and asked him about this hadith. He said: "When he (the Prophet) entered, he would say: 'O Allah, open for me the doors of Your mercy.' And when he exited, he would say: 'O Lord, open for me the doors of Your bounty.'"