Musnad Ahmad 25:855

Abu Ahmad narrated to us, Abu Bakr—meaning al-Nahshali—narrated to us, from Habib ibn Abi Thabit, from Yahya ibn al-Jazzar, from Ibn Abbas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray eight rak'ahs at night, and perform witr with three, and pray the two rak'ahs [of sunnah]. When he grew older, he moved to nine: six and three."