1. Upon entering the mosque
اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
Allaahumma iftah lee abwaaba rahmatik.
O Allah, open the gates of Your mercy for me.
Source: Sahih Muslim 713
2. Upon leaving the mosque
اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ
Allaahumma innee as'aluka min fadlik.
O Allah, I ask You of Your bounty.
Source: Sahih Muslim 713