Allah, Exalted be His praise, says: "And do not approach them until they become pure. Then when they have purified themselves, come to them from where Allah has commanded you." [Surah al-Baqarah: 222]
And Imam al-Shafi'i (may Allah have mercy on him) said: "It has been said, and Allah knows best, that (yathurna means) they become pure from menstruation, and when they have purified themselves with water."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / Q1481