It is narrated from Hazrat Buraidah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Witr prayer is obligatory, and whoever does not perform Witr is not one of us. He said this three times.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 23019
Hadith Gradingحکم دارالسلام:حسن لغيره، وهذا إسناد حسن فى المتابعات والشواهد من أجل عبيد الله بن عبدالله، والحسن بن يحيى فيه نظر، لكنه توبع