Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

that Isma'il ibn Ibrahim told him that he then met Abdullah ibn Hasan at Makkah and asked him about this hadith. He said that when the Prophet entered the mosque, he would say: (O my Lord! Open for me the doors to Your mercy). And when he come out, he would say: (O Lord! Open for me the doors to your abundance). [Ahmed 26478, Ibn e Majah 771]