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)."
Sahih al-Bukhari Book 62, Hadith 109
← Previous Hadith My brother and I came from Yemen, and for some time we continued to consider `Abdullah bin Mas`ud as one of the members of the family of the Prophet (ﷺ) because we used to see him and his mother going in the house of the Prophet (ﷺ) very often.

Sahih al-Bukhari Book 62, Hadith 108 Next Hadith → Somebody said to Ibn `Abbas, "Can you speak to the chief of the believers Mu`awiyah, as he does not pray except one rak`a as witr?" Ibn `Abbas replied, "He is a Faqih (i.e. a learned man who can give religious verdicts) ."

Sahih al-Bukhari Book 62, Hadith 110