Musannaf Ibn Abi Shayba 3:4337

6714. Muhammad ibn Ubayd narrated to us, from Khalid al-Sulami, from Ibn Sirin, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Maghrib prayer is the Witr (odd-numbered prayer) of the day, so perform Witr for the night prayer."