Because Allah the Exalted has said: "And do not have intercourse with your wives while you are in retreat (i'tikaf) in the mosques. These are the limits set by Allah, so do not approach them. Thus Allah makes His commandments clear to the people so that they may guard themselves (from sin)."

Narrated `Abdullah bin `Umar: Allah's Apostle used to practice I`tikaf in the last ten days of the month of Ramadan.