Musnad Abu Ya'la al-Mawsili 150:153

'Abd al-A'la narrated to us, 'Abd Allah ibn Dawud narrated to us, he said: I heard Hisham ibn 'Urwah, from his father—as Abu Yahya thinks—from 'Aishah: That the Prophet (peace and blessings of Allah be upon him) performed Witr with five rak'ahs, and he did not sit except in the last of them. He stood throughout all of them, except for the fifth. Dawud described it.