It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The Maghrib prayer is the witr of the day, so you should also perform the witr of the night."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4847