Muawiya offered one rak`a witr prayer after the `Isha prayer, and at that time a freed slave of Ibn `Abbas was present. He (i.e. the slave) went to Ibn `Abbas (and told him that Muawiya offered one rak`a witr prayer). Ibn `Abbas said, "Leave him, for he was in the company of Allah's Messenger (ﷺ)."
Reference: Sahih al-Bukhari 3764 In-book : Book 62, Hadith 111 USC-MSA web (English) : Vol. 5, Book 57, Hadith 108 (deprecated)
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