Muhammad bin Yahya bin Habban narrates from Ibn Muhayriz (may Allah have mercy on him) that a man of Banu Kinanah, of the clan of Banu Mudlij, met a man of the Ansar who was called Abu Muhammad. He asked him about Witr, and Abu Muhammad said: It is obligatory. The Kinani said: I met Ubadah bin al-Samit (may Allah be pleased with him) and mentioned this to him, and he said: Abu Muhammad has erred, for I heard the Prophet (peace and blessings be upon him) say: "Allah has made five prayers obligatory upon His servants. Whoever performs these five prayers without neglecting any of them, Allah has a promise for that servant that He will admit him into Paradise; and whoever does not perform these five prayers, Allah has no promise for him: if He wills, He will punish him, and if He wills, He will have mercy on him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4451