Sahih al-Bukhari 993
صحيح البخاري#993SahihNarrated `Abdullah bin `Umar:The Prophet (ﷺ) said, "Night prayer is offered as two rak`at followed by two rak`at and so on, and if you want to finish it, pray only one rak`a which will be witr for all the previous rak`at." Al-Qasim said, "Since we attained the age of puberty we have seen some people offering a three-rak`at prayer as witr and all that is permissible. I hope there will be no harm in it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 14:4