Asim bin Dmrh al-Slwly عاصم بن ضمرة السلولي الكوفي · The Book on Al-Witr

"Allah's Messenger came out to us and he said: 'Indeed Allah has assisted you with a Salat that is better for you than red camels: Al-Witr which Allah made for you between the Isha prayer till Al-Fajr has begun."

"Al-Witr is not incumbent like your obligatory prayers, but it is a Sunnah of Allah's Messenger who said: "Indeed Allah is Witr (One), and He loves Al-Witr, so perform Al-Witr O people of the Qur'an."