Musnad Abu Ya'la al-Mawsili 165:11

Narrated to us Abdullah ibn Muhammad ibn Salim, narrated to us Husayn ibn Zayd, from al-Hasan ibn Zayd, from his father, from al-Husayn ibn Ali: That the Prophet (peace and blessings of Allah be upon him) used to perform ablution, and he would wash the place of his prostration with water until he made it flow over the place of prostration.