Mujahid (may Allah have mercy on him) said regarding the saying of Allah Almighty, "And do not approach them until they become pure" [Surah al-Baqarah: 222]: Until the blood stops. And regarding "Then when they have purified themselves" [Surah al-Baqarah: 222], he said: When they have taken a ritual bath.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1482