Riyad as-Salihin

Abu Sa'id Al-Khudri (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "When you see a man frequenting the mosque, testify that he is a believer because Allah says: 'Allah's mosques are visited only by those who believe in Allah and the Last Day."'

[At-Tirmidhi].