Muawiya prayed a single rakah of witr after the night prayer, and a freedman of Ibn Abbas was present with him. The freedman went to Ibn Abbas and reported this, and Ibn Abbas said, "Leave him, for he was a companion of the Messenger of Allah ﷺ."
أوتر معاوية بعد العشاء بركعة وعنده مولى لابن عباس، فأتى ابن عباس، فقال: دعه فإنه صحب رسول الله ﷺ.
Bukhari narrated it in the Virtues of the Companions (3764) from Hasan bin Bishr, from Muafa, from Uthman bin Aswad, from Ibn Abu Mulayka, who said, then he mentioned it.And in another wording, "It was said to Ibn Abbas, 'What do you say about the Commander of the Faithful Muawiya, for he performed the witr with only one unit?' He said, 'He is a jurist.'"Bukhari narrated it in the Virtues of the Companions (3765) through another route from Ibn Abu Mulayka, with the same wording.