Musnad Ahmad 1153:904

'Affan narrated to us, he said: Hammam narrated to us, Hisham ibn 'Urwah narrated to us, he said: My father narrated to me, that 'Aishah narrated to him: "That the Messenger of Allah (peace and blessings of Allah be upon him) used to sleep, and when he woke up, he would use the siwak, then perform wudu, then pray eight rak'ahs, sitting in every two rak'ahs and performing the taslim. Then he would perform witr with five rak'ahs, not sitting except in the fifth, and not performing the taslim except in the fifth."