Sayyiduna Samurah (may Allah be pleased with him) says: Amma ba'd (to proceed): The Messenger of Allah (peace and blessings be upon him) used to order us that each of us should pray every night after the obligatory prayer, whether little or much, and make it odd (witr).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6971