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."
Reference: Sunan an-Nasa'i 362 In-book : Book 3, Hadith 14 English translation : Vol. 1, Book 3, Hadith 362
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