What is the evidence for the Witr prayer?

The basis for the Witr prayer is the words and practices of the Prophet Muhammad (peace be upon him). The Prophet (peace be upon him) said, “Witr is a duty upon every Muslim.” (Abu Dawud, Tefrî‘u ebvâbi’l-witr, 3 [1422]; Nasa’i, Qiyâm al-layl, 40 [1710]). He also recommended and encouraged that the last prayer of the night be performed with an odd number of units (rak‘ahs) (Bukhari, Salat, 84 [472-473]; Muslim, Salat al-Musafirin, 148 [749]).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu