سورة Al-Baqarah ( The Cow ) - الآية 222

And they ask thee of menstruation. Say thou: it is a pollution, so keep away from women during menstruation, and go not in unto them until they have purified themselves. Then when they have thoroughly purified themselves, go in unto them as Allah hath commanded you. Verily Allah loveth the penitents, and loveth the purifiers of themselves.