Abu Mijlaz reported: 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.
Explanation & Benefits
Shaykh Maulana Abdul Aziz Alvi
Hadith Commentary: Benefits and Issues: From the aforementioned narration of Ibn Umar radi Allahu anhuma and Ibn Abbas radi Allahu anhuma, it is established that in the end of Witr, only one rak‘ah will be performed. And to say, in the presence of an explicit hadith, that it is a single rak‘ah joined to a two-unit prayer, is the height of prejudice.
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 1759