Musannaf Ibn Abi Shayba 3:4427

Shababa ibn Sawwar narrated to us, he said: Ibn Abi Dhi'b narrated to us, from al-Zuhri, from 'Urwah, from 'Aishah, that the Prophet (peace and blessings of Allah be upon him) used to perform Witr with one Rak'ah. And he used to speak between the two Rak'ahs and the [single] Rak'ah.