Musnad Ahmad 1153:2339

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Hisham ibn 'Urwah ibn al-Zubayr and Muhammad ibn Ja'far ibn al-Zubayr narrated to me, both of them narrated to me from 'Urwah ibn al-Zubayr, from 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, she said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray thirteen rak'ahs at night, including his two rak'ahs after the dawn [prayer] before the morning [prayer], eleven rak'ahs of the night, six of them [performed] two by two, and he would perform witr with five, not sitting during them."