Abdullah al-Sunabihi said: Abu Muhammad claimed that the witr is obligatory. So Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) said: Abu Muhammad has spoken falsely; I bear witness that I heard the Messenger of Allah (peace and blessings be upon him) saying: "Allah Almighty has made five prayers obligatory. Whoever performs their ablution well, offers them on time, completes their bowing and their prostrations, and offers them with humility and devotion, then it is a covenant upon Allah Almighty that He will forgive such a servant; and whoever does not do so, there is no covenant upon Allah Almighty for him: if He wills, He forgives him, and if He wills, He punishes him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6500