Musannaf Ibn Abi Shayba 3:4338

Ibn Numayr narrated to us, he said: Al-A’mash narrated to us, from Malik ibn al-Harith, from ‘Abd al-Rahman ibn Yazid, he said: ‘Abdullah said: The Witr is three [rak'ahs], like the Maghrib prayer; it is the Witr of the day.