Musnad Abu Ya'la al-Mawsili 150:289

Hammad ibn al-Sari narrated to us, Abu al-Ahwas narrated to us, from al-A'mash, from Ibrahim, from al-Aswad, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray nine rak'ahs at night.