When entering the mosque, say -

أَعُوْذُ بِاللَّهِ الْعَظِيمِ وَبِوَجْهِهِ الْكَرِيمِ وَسُلْطَانِهِ الْقَدِيمِ مِنَ الشَّيْطَانِ الرَّجِيمِ

A‛oodhu Billaahil ‘Azeem, Wabi Wazhihil Kareem, Wa Sultaanihil Qadeem Minash Shaytaanir rajeem

I seek refuge in Allah, the Magnificent, and in His noble face, and in his eternal domain, from the accursed Devil. Uqbah (RA) said, ‘When someone recites this du’a, the devil says, ‘This man escaped from me (evil and intrigue) for the whole day.

Reference: Sahih. Abu Dawud: 466