Sunan Ibn Majah Book 1, Hadith 105
One of the wives of the Prophet took a bath to cleanse herself from sexual impurity, then the Prophet performed ablution and had a bath with the water left over from her ablution."
"One of the wives of the Prophet took a bath from a large vessel, then the Prophet came and had a bath or ablution, and she said: 'O Messenger of Allah, I was sexually impure.' He said: 'Water does not become impure.'"
the Prophet performed ablution with the water left over after she had taken a bath to cleanse herself from sexual impurity.