The Messenger of Allah ﷺ said, "Whoever does not pray witr is not one of us."
حدثنا وكيع قال حدثنا خليل بن مرة عن معاوية بن قرة
قال رسول الله ﷺ من لم يوتر فليس منا
Its chain of transmission is hasan. Khalil bin Murrah is disputed among the scholars. Abu Zura said about him, "A righteous shaykh," and Haythami said likewise in 2/240, mentioning the disagreement about him.The hadith was narrated by Ibn Abu Shayba in 2/297, in the chapter on prayer, from those who said, "Witr is obligatory."