Sayyidah Aishah Siddiqah (may Allah be pleased with her) narrates: The Prophet (peace and blessings be upon him) said: "Like the three rak'ahs of Maghrib, the witr is three rak'ahs."
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat; its chain contains a narrator, Abu Bahr al-Bakrawi, about whom much has been said.