Muhammad bin Yahya bin Habban narrates from Ibn Muhayriz that a man of Banu Kinanah, who was called al-Mukhdaji, heard a man in Syria who was called Abu Muhammad saying that Witr is obligatory (wajib). Al-Mukhdaji said: I went to Ubadah bin al-Samit (may Allah be pleased with him) and put this matter before him while he was on his way to the mosque, and I told him what Abu Muhammad had said. Ubadah (may Allah be pleased with him) said: Abu Muhammad has erred. I heard the Prophet (peace and blessings be upon him); he (peace and blessings be upon him) was saying: "Allah has made five prayers obligatory upon His servants. Whoever performs these five prayers and does not neglect them by regarding them lightly, Allah has a promise to that servant that He will admit him into Paradise. Whoever does not come to these five prayers, Allah has no promise to him: if He wills, He will punish him, and if He wills, He will admit him into Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4450