Abu Dawud Book 1, Hadith Number 68.
Chapter : The water left over after bath is not defiled.
Narated By Abdullah ibn Abbas : One of the wives of the Prophet (PBUH) took a bath from a large bowl. The Prophet (PBUH) wanted to perform ablution or take from the water left over. She said to him: O Prophet of Allah, verily I was sexually defiled. The Prophet said: Water not defiled.