The Prophet ﷺ said, "The Maghrib prayer is the witr of the day, so make your night prayer witr as well."
حدثنا يزيد أخبرنا هشام عن محمد بن سيرين
عن النبي ﷺ قال صلاة المغرب وتر النهار فأوتروا صلاة الليل
Its isnad is sahih. Hisham is Ibn Hassan. I have not found this hadith with this wording anywhere other than this place. In Sunan al-Kubra by Bayhaqi, 2:26, there is a hadith containing a story with a similar meaning. See also 4710.