"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 Abi 'Adi mentioned. And Allah Most High, knows best.
Sunan an-Nasa'i Book 1, Hadith 217
← Previous Hadith "If it is menstrual blood then it is blood that is black and recognizable, so stop praying, and if it is other than that then perform Wudu', for it is just a vein."

Sunan an-Nasa'i Book 1, Hadith 216 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 perform Wudu'. That is a vein and is not menstruation.'" It was said to him: "What about Ghusl?' He (ﷺ) said: "no one doubts that." Abu 'Abdur-Rahman said: "I do not know anyone who mentioned 'and perform Wudu' in this Hadith except Hammad bin Zaid, for some others have reported it from Hisham, and they did not mention 'and perform Wudu' in it."

Sunan an-Nasa'i Book 1, Hadith 218