Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "When you see someone going regularly to the mosque, bear witness to his faith." Allah Almighty says: "The mosques of Allah are maintained only by those who believe in Allah and the Last Day." [Surah al-Tawbah: 18]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4988