It is not impure. It is one of those who roam around among you
عن كبشة بنت كعب بن مالك -وكانت تحت ابن أبي قتادة-: أن أبا قتادة دخل فسَكَبَتْ له وَضُوءًا، فجاءت هرة فشربت منه، فأصغى لها الإناء حتى شربت، قالت كبشة: فرآني أنظر إليه، فقال: أتعجبين يا ابنة أخي؟ فقلت: نعم، فقال: إن رسول الله صلى الله عليه وسلم قال: «إنها ليست بنجس، إنها من الطوافين عليكم والطوافات».
Explanation
The Hadīth relates that Abu Qatādah had started making ablution when a cat came in - and cats frequently enter people's homes and mix with them. He tilted the vessel for it so it could drink from the water of ablution. Kabshah, his niece, was surprised at what he did, given that the water was prepared for ablution and it must be pure and purifying.
So he related to her the Hadīth in which the Prophet (may Allah's peace and blessings be upon him) said that cats are not impure and thus do not affect the purity of water, because it is an animal that always mixes with people.
Source
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