It is narrated from Umm Salamah (may Allah be pleased with her) that Fatimah bint Abi Hubaysh suffered from istihadah (irregular bleeding), and Umm Salamah (may Allah be pleased with her) asked the Messenger of Allah (peace and blessings be upon him) on her behalf. The Messenger of Allah (peace and blessings be upon him) said: "This is not menstruation; it is only a vein." Then he (peace and blessings be upon him) commanded her to leave the prayer during the days of her quru' or of her menstruation, then to take a bath and pray, and if the blood overwhelmed her, to bind herself with a cloth.
The Shaykh (al-Bayhaqi) said: This wording which they used as evidence has been narrated in hadiths that we mentioned in the Book of Menstruation, and those hadiths themselves differ: some narrators said in them "the days of her aqra'", and some said "the days of her menstruation" or something with that meaning. All of that comes from the narrators, each of them expressing it as it occurred to him; and the authentic hadiths agree on expressing it as "the days of menstruation", without the word aqra'. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15393