Musnad Abu Ya'la al-Mawsili 150:295

Zakariya ibn Yahya narrated to us, Waki’ and Abdullah ibn Dawud and ‘Atham ibn ‘Ali and ‘Ubaydullah ibn Musa narrated to us, from al-A’mash, from Habib ibn Abi Thabit, from ‘Urwah, from ‘Aishah, she said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The woman experiencing istihada (irregular vaginal bleeding) shall pray, even if the blood drips onto the mat." Ibn Dawud said: "Drips (qatan)."