Duaa-When entering the Masjid

When entering the Masjid recite Durood and Salaam first and thereafter recite:
رَبِّ اغْفِرْ لِي ذُنُوبِي وَافْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
"O Lord, forgive my sins and open the doors of mercy for me".

(Mishkat)
Alternatively recite
اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
"O Allah, open the doors of mercy".

(Mishkat)