The Messenger of Allah ﷺ forbade using the leftover water from a woman's purification.
حدثنا محمود بن غيلان قال حدثنا وكيع عن سفيان عن سليمان التيمي عن أبي حاجب
نهى رسول الله ﷺ عن فضل طهور المرأة
Hasan. Ahmad reported it (5/66) through Sulayman Taymi with the same wording. It is documented in Nail al-Maqsud, hadith 82, and in Tas-heel al-Hajah fi Takhreej Ibn Majah, hadith 373, and elsewhere. See also the following hadith. There is also a report on this subject from Abdullah bin Sarjas [Ibn Majah, hadith 374].