"That is a vein, so when your period comes, do not pray, and when your period is over, purify yourself and pray in between one period and the next." This is evidence that Al-Aqra' is menstruation. Abu 'Abdur-Rahman said: Hisham bin 'Urwah reported this Hadith from 'Urwah, and he did not mention what Al-Mundhir mentioned in it.
Sunan an-Nasa'i Book 1, Hadith 212
← Previous Hadith "That is not menstruation, rather it is a vein. Tell her not to pray for the amount of time that her period used to last, then let her perform Ghusl and pray.' She used to perform Ghusl for every prayer.

Sunan an-Nasa'i Book 1, Hadith 211 Next Hadith → Fatimah bint Abi Hubaish came to the Messenger of Allah (ﷺ) and said: "I am a woman who suffers from Istihadah (non-menstrual vaginal bleeding) and I never become pure. Should I stop praying?" He said: "No, that is a vein, it is not menstruation. When you period comes, stop praying, and when it goes, wash the blood from yourself and pray."

Sunan an-Nasa'i Book 1, Hadith 213