When someone leaves the mosque, he should greet the Prophet (ﷺ) and say:

اَللَّهُمَّ اعْصِمْنِى مِنَ الشَّيْطَانِ الرَّجِيْمِ

Allaahumma aa'simni minash shaitaanir rajeem

O Allah, protect me from Satan the outcast.

Reference: Sahih (Albani). Ibn Majah: 773