I asked Ibn 'Abbas about the Witr prayer. He said: I heard the Messenger of Allah (ﷺ) as saying: It is a rak'ah at the end of the night prayer.
Sahih Muslim Book 6, Hadith 188
← Previous Hadith Witr is a rak'ah at the end of the night prayer.

Sahih Muslim Book 6, Hadith 187 Next Hadith → A person called (the attention) of the Messenger of Allah (ﷺ) as he was in the mosque, and said: Messenger of Allah, how should I make the rak'ahs of the night prayer an odd number? Upon this the Messenger of Allah (may peace he upon him) said: He who prays (night prayer) he should observe it in pairs, but if he apprehends the rise of morning, he should observe one rak'ah; that would make the number odd (for the rak'ahs) observed by him. This was narrated by Abu Kuraib 'Ubaidullah b. 'Abdullah and Ibn 'Umar did not make mention of it.

Sahih Muslim Book 6, Hadith 189