Mosques and places of Prayer - Section 1 Mishkat al-Masabih 703
Arabic hadith text
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
Translation
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.