It 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

English translation · Sunan an-Nasa'i