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

from his mother Fatimah bint Husayn who reported on the authority of her grandmother Sayyidah Fatimah Kubra that when Allah's Messenger entered a mosque, he invoked blessings on himself and made this supplication: (O my Lord! Forgive me my sins, and open for me the doors to your mercy). And, when he came out of the mosque, he again invoked blessings on himself and made this prayer: (O my Lord! Forgive me my sins, and open for me the doors to your abundance).