اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
O Allah, open for me the gates of Your mercy.
Source: Muslim
Said when entering the masjid.
أَعُوذُ بِاللَّهِ الْعَظِيمِ، وَبِوَجْهِهِ الْكَرِيمِ، وَسُلْطَانِهِ الْقَدِيمِ، مِنَ الشَّيْطَانِ الرَّجِيمِ
I seek refuge in Allah the Magnificent, by His Noble Face and by His eternal authority, from Satan the rejected.
Source: Abu Dawud
اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ
O Allah, I ask You from Your bounty.
Source: Muslim
Said when leaving the masjid.
اللَّهُمَّ صَلِّ عَلَى مُحَمَّدٍ وَسَلِّمْ
O Allah, send prayers and peace upon Muhammad.
Source: Ibn Majah
From the adhkar of entering and leaving the masjid.