Musnad Ahmad 1012:39

Husayn ibn Muhammad narrated to us, Muhammad ibn Mutarrif narrated to us, from Zayd ibn Aslam, from ‘Ata’ ibn Yasar, from ‘Abdullah al-Sanabihi, who said: Abu Muhammad claimed that the Witr prayer is obligatory. ‘Ubadah ibn al-Samit said: Abu Muhammad has lied. I bear witness that I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "Five prayers which Allah has made obligatory upon His servants. Whoever performs their ablution well, prays them at their proper times, and completes their bowing, prostration, and humility, has a covenant with Allah that He will forgive him. And whoever does not do so, has no covenant with Allah; if He wills, He will forgive him, and if He wills, He will punish him."