Musnad Ahmad 1153:672

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Abu al-Tayyah, he said: I heard Sulayman ibn Murthad—or Mazid—narrating from 'Aishah, that she said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray nine rak'ahs at night."