Sunan Ibn Majah, Hadith 802 ⓘ
It was narrated from Abu Sa'eed that: The Messenger of Allah said: "If you see a man frequenting the mosques, then bear witness to his faith. Allah says: 'The mosques of Allah shall be maintained only by those who believe in Allah and the Last Day. [At-Taubah: 18]'"
حَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا رِشْدِينُ بْنُ سَعْدٍ، عَنْ عَمْرِو بْنِ الْحَارِثِ، عَنْ دَرَّاجٍ، عَنْ أَبِي الْهَيْثَمِ، عَنْ أَبِي سَعِيدٍ، عَنْ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ قَالَ " إِذَا رَأَيْتُمُ الرَّجُلَ يَعْتَادُ الْمَسَاجِدَ فَاشْهَدُوا لَهُ بِالإِيمَانِ قَالَ اللَّهُ تَعَالَى {إِنَّمَا يَعْمُرُ مَسَاجِدَ اللَّهِ مَنْ آمَنَ بِاللَّهِ} الآيَةَ .
also referenced under: Book 4, Hadith 68 (In-Book) & Volume 1, Book 4, Hadith 802 (DarusSalam)