Musnad Abu Ya'la al-Mawsili 150:233

Hannad ibn al-Sari narrated to us, Abu al-Ahwas narrated to us, from al-A'mash, from Ibrahim, from al-Aswad, from Aisha, she said: The Messenger of Allah (may Allah bless him and grant him peace) used to pray nine rak'ahs at night.