Ata' bin Yasar (may Allah have mercy on him) and Abdullah al-Sunabihi (may Allah have mercy on him) said that Abu Muhammad held the view that the witr is wajib (obligatory).
Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) said: Abu Muhammad is mistaken. I bear witness that I heard the Messenger of Allah (peace and blessings be upon him) saying: "Allah, the Lord of Might, has made five prayers obligatory. Whoever performs ablution well, offers them on time, performs their bowing calmly and observes humility and devotion in them, has a promise from Allah Almighty that He will forgive him; and whoever does not do so has no promise from Allah Almighty: if He wills He will forgive him, and if He wills He will punish him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3166