Muhammad bin Abi 'Adi محمد بن أبي عدي · The Book of Menstruation and Istihadah

"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."

"Menstrual blood is blood that is black and recognizable, so if it is like that, then stop praying, and if it is otherwise, then perform Wudu' and pray." Abu 'Abdur-Rahman said: Others reported this Hadith, and none of them mentioned what Ibn 'Adi mentioned, and Allah knows best.