Abu Dawud Book 1, Hadith Number 75.
Chapter : The left over of a cat.
Narated By Abu Qatadah : Kabshah, daughter of Ka’b ibn Malik and wife of Ibn AbuQatadah, reported: AbuQatadah visited (me) and I poured out water for him for ablution. A cat came and drank some of it and he tilted the vessel for it until it drank some of it. Kabshah said: He saw me looking at him; he asked me: Are you surprised, my niece? I said: Yes. He then reported the Messenger of Allah (PBUH) as saying: It is not unclean; it is one of those (males or females) who go round among you.