مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 703
Prayer
Mosques and places of Prayer - Section 1
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
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 Your mercy, “and when he goes out he should say “O God, I ask You out of Your abundance.” Muslim transmitted it.
Mishkat al-Masabih, 703صَحِيح (الألباني)