Mishkat al-Masabih #673
Prayer
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
عَنْ أَبِي سَعِيدٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " إِذَا رَأَيْتُمُ الرَّجُلَ يَتَعَاهَدُ الْمَسْجِد فَاشْهَدُوا لَهُ بِالْإِيمَان فَإِن الله تَعَالَى يَقُولُ (إِنَّمَا يَعْمُرُ مَسَاجِدَ اللَّهِ مَنْ آمَنَ بِاللَّه وَالْيَوْم الآخر وَأقَام الصَّلَاة وَآتى الزَّكَاة)
رَوَاهُ التِّرْمِذِيّ وَابْن مَاجَه والدارمي
Abu Sa'id al-Khudri reported God’s Messenger as saying, “When you see a man frequenting the mosque, testify that he has faith, for God says, ‘God’s mosques are visited only by those who believe in God and the last day.”*
* Al-Qur'an; 9:18
Tirmidhi, Ibn Majah and Darimi transmitted it.
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About Mishkat al-Masabih
This hadith is #673 from Mishkat al-Masabih, in the Book of Prayer.