When entering the mosque, say -

اَللَّهُمَّ اعْصِمْنِي مِنَ الشَّيْطَانِ الرَّجِيمِ

Allaahumma`simni min ash-shaytaanir-rajeem

O Allah, protect me from Satan the outcast.

Reference: Sahih. Ibn Majah: 773