Dua upon entering the mosque

ِأَعُوْذُ بِاللَّـهِ الْعَظِيْـمِ وَبِوَجْهِـهِ الْكَرِيـمِ وَسُــلْطَـانِهِ الْقَدِيْـم
ِمِنَ الشّيْـطَانِ الرَّجِـيْم
ِبِسْـمِ اللَّـهِ، وَالصَّلَاةُ وَالسَّلَامُ عَلَى رَسُوْلِ الله
اللَّهُـمَّ افْتَـحْ لِي أَبْوَابَ رَحْمَتـِك

A'ūdhu billāhi l-'aẓīm, wa bi-wajhihi l-karīm, wa sulṭānihi l-qadīm, min aš-šayṭāni r-rajīm. Bismi l-lāh, wa aṣ-ṣalātu wa as-salāmu 'alā rasūli l-lāh, allāhumma ftah lī abwāba raḥmatik.

Translation

I take refuge with Allah, The Supreme and with His Noble Face, and His eternal authority from the accursed devil. In the name of Allah, and prayers and peace be upon the Messenger of Allah. O Allah, open the gates of Your mercy for me.

When to recite

Upon entering the mosque.

Source / Reference

Hisnul Muslim 20