Musnad Abu Ya'la al-Mawsili 217:18

Zuhayr narrated to us, Hashim ibn al-Qasim narrated to us, Shu'bah narrated to us, from al-Hakam, who said: I said to al-Qasim, "Perform Witr with three [rak'ahs] when the mu'adhdhin gives the adhan, then go out to the prayer." He said, "Do not pray except with five or seven." Al-Hakam said: So I informed Mujahid and Yahya ibn al-Jazzar of that, and they both said to me, "Ask him from whom this is." He said, "From the trustworthy [source], from 'A'ishah and Maymunah, from the Prophet, peace and blessings of Allah be upon him."