Musnad Ahmad 1153:510

Ishaq narrated to us, he said: Layth narrated to me, he said: Ibn Shihab narrated to me, from Urwah ibn al-Zubayr, from Aisha, that she said: Umm Habibah bint Jahsh sought a legal ruling from the Messenger of Allah, may Allah bless him and grant him peace, and she said: "I am experiencing *istihada* (prolonged non-menstrual bleeding)." He said: "That is only a vein, so perform ghusl, then pray." So she used to perform ghusl for every prayer. Ibn Shihab said: "The Prophet, may Allah bless him and grant him peace, did not command her to perform ghusl for every prayer; she only did that herself."