اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
Allahumma-ftah li abwaba rahmatik
Allaahumma ii fur albaabbada naxariistada.
Recite when entering the masjid
The Prophet (PBUH) would say this when entering the masjid, asking Allah to open the doors of His mercy.