وَسُؤْرُ الْهِرَّةِ وَالدَّجَاجَةِ الْمُخَلَّاةِ وَسِبَاعِ الطُّيُورِ وَمَا يَسْكُنُ فِي الْبُيُوتِ مِثْلَ الْحَيَّةِ وَالْفَأْرَةِ: مَكْرُوهٌ
Translation: The leftover water of a cat, an unconfined hen, birds of prey, and house-dwelling creatures such as a snake or mouse is disliked.
This disliked category does not automatically mean impure water. Contact, habitual feeding behavior, and the possibility of contamination are weighed separately from definite impurity.
Using such leftover water for wudu must be distinguished from a case where actual filth is known to have entered it.