Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Taharah (Purification)

wife of the son of Sayyidina Abu Qatadah (RA) narrated," Abu Qatadah visited us and I poured out water in a vessel for ablution he may make. A cat came and began to drink it. He tilted the vessel for it till she drank her fill. He saw me looking at him and asked if I was surprised. When I said that I was, he said that Allah's Messenger (SAW) had said that it was not impure (or unclean) and it is one of those that go round us. [Ahmed 22591, 2264]