Mishkat al-Masabih 703
Abu Usaid reported God's Messenger as saying:
When any of you enters the mosque he should say, “O God, open to me the gates of Thy mercy, “and when he goes out he should say “O God, I ask Thee out of Thine abundance.” 

Muslim transmitted it.
وَعَنْ أَبِي أُسَيْدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: 
" إِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلْيَقُلِ: اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ. وَإِذَا خَرَجَ فَلْيَقُلِ: الله إِنِّي أَسأَلك من فضلك ". رَوَاهُ مُسلم