"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.
Sunan an-Nasa'i Book 3, Hadith 15
← Previous Hadith "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."

Sunan an-Nasa'i Book 3, Hadith 14 Next Hadith → "Fatimah bint Abi Hubaish suffered from Istihadah and she asked the Prophet (ﷺ): 'O Messenger of Allah, I suffer from Istihadah and I do not become pure; should I stop praying?' The Messenger of Allah (ﷺ) said: 'That is a vein and is not menstruation. When your period comes, stop praying, and when it goes wash the traces of blood from yourself and do Wudu'. That is a vein and is not menstruation.'" It was said to him (one of the narrators): "What about Ghusl?" He said: "No one is in doubt about that."

Sunan an-Nasa'i Book 3, Hadith 16