Whenever the Messenger of Allah (ﷺ) entered the mosque he would say Durood. Then Say:

اَللَّهُمَّ اغْفِرْ لِي ذُنُوْبِي وَافْتَحْ لِي أَبْوَابَ رَحْمَتِكَ

Rabbig-firli dhunubi waftah li abwaaba rahmatik

O Allah, forgive me my sins and open to me the gates of Your mercy.

Reference: Sahih (Albani). Ibn Majah: 771