It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that one of the wives of the Prophet (peace and blessings be upon him) bathed in a tub. The Prophet (peace and blessings be upon him) came to perform ablution or take a bath from it. She said: O Messenger of Allah! I was in a state of major impurity (junub). The Messenger of Allah (peace and blessings be upon him) said: "Indeed, water does not become impure."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1265