Sunan an-Nasa'i 362
سنن النسائي#362SahihIt was narrated from Fatimah bint Abi Hubaish that she suffered from Istihadah and the Messenger of Allah (ﷺ) said to her:"If it is menstrual blood then it is blood that is black and recognizable, so stop prying, and if it is other than that then perform Wudu', for it is just a vein
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahHasan Sahih
Al-AlbaniHasan Sahih
Zubair Ali ZaiSahih
In-book reference: 3:14