Musnad Ahmad 1311:1

Yunus ibn Muhammad narrated to us, he said: Layth narrated to us, from Yazid ibn Abi Habib, from Bukayr ibn Abdullah, from al-Mundhir ibn al-Mughirah, from Urwah ibn al-Zubayr, that Fatimah bint Abi Hubaysh narrated to him that she came to the Messenger of Allah, peace and blessings of Allah be upon him, and complained to him about the blood. The Messenger of Allah, peace and blessings of Allah be upon him, said: "That is only a vein. So observe: when your period comes, do not pray. Then, when the period has passed, purify yourself, then pray between one period and the next."