Musnad Ahmad 25:1412

Mu'awiyah ibn Hisham narrated to us, Sufyan narrated to us, from Habib ibn Abi Thabit, from Muhammad ibn Ali, from his father, from his grandfather, from the Prophet, peace and blessings of Allah be upon him: "That he stood up at night, then used the siwak, then prayed two rak'ahs, then slept. Then he stood up, used the siwak, performed wudu, and prayed two rak'ahs, until he had prayed six. Then he performed witr with three, and prayed two rak'ahs."