It was narrated from Kabshah bint Ka'b, who was married to one of the sons of Ab Qatadah, that:
She poured water for Abu Qatadah to perform ablution. A cat came and drank the water, and he tilted the vessel for it. She started looking at it (in surprise) and he said: "O daughter of my brother, do you find it strange? The Messenger of Allah said: 'They (cats) are not impure, they are of those who go around among you.'"
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا زَيْدُ بْنُ الْحُبَابِ، أَنْبَأَنَا مَالِكُ بْنُ أَنَسٍ، أَخْبَرَنِي إِسْحَاقُ بْنُ عَبْدِ اللَّهِ بْنِ أَبِي طَلْحَةَ الأَنْصَارِيُّ، عَنْ حُمَيْدَةَ بِنْتِ عُبَيْدِ بْنِ رِفَاعَةَ، عَنْ كَبْشَةَ بِنْتِ كَعْبٍ، وَكَانَتْ، تَحْتَ بَعْضِ وَلَدِ أَبِي قَتَادَةَ أَنَّهَا صَبَّتْ لأَبِي قَتَادَةَ مَاءً يَتَوَضَّأُ بِهِ فَجَاءَتْ هِرَّةٌ تَشْرَبُ فَأَصْغَى لَهَا الإِنَاءَ فَجَعَلْتُ أَنْظُرُ إِلَيْهِ فَقَالَ يَا ابْنَةَ أَخِي أَتَعْجَبِينَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ إِنَّهَا لَيْسَتْ بِنَجَسٍ هِيَ مِنَ الطَّوَّافِينَ أَوِ الطَّوَّافَاتِ ‏"‏ ‏.‏
GradeSahih (Darussalam)
English reference : Vol. 1, Book 1, Hadith 367
Arabic reference : Book 1, Hadith 397