Its excellence and justification: The witr prayer is one that the Prophet sallallahu alehi wasallam practiced and which he encouraged others to practice. As such, praying witr comes under as-sunnah al-mu’akkadah.
‘Ali says: “The witr prayer is not required like your obligatory prayers, but the Prophet would perform the witr prayer and say: ‘O you people [followers] of the Qur’an, perform the witr prayer, for Allah is one and He loves the witr.'” This is related by Ahmad, an-Nasa’i, Abu Dawud, Ibn Majah, at-Tirmizhi who calls it hasan, and al-Hakim who grades it sahih.
The opinion of Abu Hanifah that the witr prayer is obligatory is a weak opinion. Ibn al-Munzhir says: “I don’t know anyone who agrees with Abu Hanifah on this point.”
Ahmad, Abu Dawud, An-Nasa’i, and Ibn Majah record that Al-Mukhdaji [a person of the Kinana tribe] heard from one of the Ansar, nicknamed Abu Muhammad, that the witr prayer is obligatory. He went to ‘Ibadah ibn as-Samit and mentioned to him what Abu Muhammad had said. ‘Ibadah observed: “Abu Muhammad is mistaken for I heard the Messenger of Allah say: ‘Five prayers are ordained by Allah for his slaves. Whoever fulfils them properly without any shortcoming, he will have a pact with Allah that He will admit him into paradise. Whoever does not do them, he will have no pact with Allah, and if He wills He may punish him and if He wills He may forgive him.'”
Also al-Bukhari and Muslim record from Talhah ibn ‘Ubaidullah that the Prophet said: “Five prayers during the day and night have been prescribed by Allah.” Hearing this a bedouin asked the Prophet: “Is there anything else upon me [in the way of prayer]?” The Prophet said: “No, unless you want to do more voluntarily.”