The prohibition of that (i.e., performing ablution with the leftover water)
حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ إِسْرَائِيلَ، عَنْ أَبِي إِسْحَاقَ، عَنِ الْحَارِثِ، عَنْ عَلِيٍّ، قَالَ كَانَ النَّبِيُّ ـ صلى الله عليه وسلم ـ وَأَهْلُهُ يَغْتَسِلُونَ مِنْ إِنَاءٍ وَاحِدٍ وَلاَ يَغْتَسِلُ أَحَدُهُمَا بِفَضْلِ صَاحِبِهِ .
It was narrated that 'Ali said: "The Prophet (ﷺ) and his wife would take a bath from one vessel, but neither of them would have a bath with the leftover water of the other." (Daif)