"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
← Previous Hadith "I said to the Prophet (ﷺ) that I was suffering from Istihadah. He said: 'Do not pray during the days of your period, then perform Ghusl and delay Zuhr and bring Maghrib and bring 'Isha' forward and pray them together, and perform Ghusl for Fajr."

Sunan an-Nasa'i Book 3, Hadith 13 Next Hadith → "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