Musnad Ahmad 1261:1

Yunus ibn Muhammad narrated to us, he said: Layth ibn Sa'd narrated to us, he said: Yazid ibn Abi Habib narrated to me, 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 Prophet, 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 to her: "That is only a vein, so observe. When your period comes, do not pray, and when the period passes, purify yourself, then pray between one period and the next."