Sunan Ibn Majah Book 1, Hadith 103
The Messenger of Allah said: "Cats do not invalidate the prayer, because they are one of the things that are useful in the house."
"The Messenger of Allah and I used to perform ablution from a single vessel, when the cat had drunk from it beforehand."
"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.'"