Hadith on Mosques: Supplication for entering and exiting the Masjid

Abu Usaid reported: The Messenger of Allah, peace and blessings be upon him, said, “When one of you enters the mosque, then let him say: O Allah, open for me the doors of your mercy. When he leaves, then let him say: O Allah, verily, I ask you for your favor.”
Source: Sahih Muslim 713
Grade: Sahih (authentic) according to Imam Muslim

عَنْ أَبِي أُسَيْدٍ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا دَخَلَ أَحَدُكُمْ الْمَسْجِدَ فَلْيَقُلْ اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ وَإِذَا خَرَجَ فَلْيَقُلْ اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ

713 صحيح مسلم كِتَاب صَلَاةِ الْمُسَافِرِينَ وَقَصْرِهَا باب مَا يَقُولُ إِذَا دَخَلَ الْمَسْجِدَ