When someone leaves the mosque, he should send a durood on the Prophet (ﷺ) and say:

اَللَّهُمَّ اغْفِرْ لِي ذُنُوْبِي وَافْتَحْ لِي أَبْوَابَ فَضْلِكَ

Allaahummaghfirli dhunubi, waftahli abwaba fadlik

O Lord pardon my sins, and open the gates of Your blessings for me.

Reference: Sahih (Albani). Ibn Majah: 771