The Repentance - 9:28
O you who believe, the idolaters are unclean; let them not approach the Holy Mosque after this year. If you fear poverty, Allah will, if He wills, enrich you with His bounty. Allah is All-Knowing, All-Wise.
Why?
The Repentance/9:18
Only those who believe in Allah and the Last Day, who observe prayer, pay the zakat, and fear none but Allah, visit the mosques of Allah frequently. They are of those who find the right path.