Musnad Ahmad 31:660

And with this chain of narration, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If you see a man frequenting the mosque, then testify to his faith." Allah, the Almighty and Majestic, says: "The mosques of Allah are only to be maintained by those who believe in Allah and the Last Day" [At-Tawbah: 18].