It is narrated from Abu Ayyub al-Ansari (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Witr is a duty. Whoever likes to pray five rak'ahs of witr, let him do so; whoever likes to pray three rak'ahs of witr, let him do so; whoever likes to pray one rak'ah of witr, let him do so; and whoever is not able even to do that should perform it by gestures."
(b) A group of the Companions held that the voluntary prayer or witr may be offered as a single separate rak'ah, and among them is Umar bin al-Khattab (may Allah be pleased with him).
Hammad bin Zayd and Abd al-Razzaq narrated it from Ma'mar as a statement of Abu Ayyub (mawquf), and likewise a group narrated it from al-Zuhri as a statement of Abu Ayyub. Muhammad bin Yahya said: This hadith, in the narration of Yunus, al-Zubaydi, Ibn Uyaynah, Shu'ayb, Ibn Abi Ishaq and Abd al-Razzaq from Ma'mar, is more likely not to be marfu' (attributed to the Prophet), and there is doubt in the mind about the narration of the others along with the narration of Wuhayb from Ma'mar. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4780