Sahih Bukhari Volume 5, Book 57, Hadith Number 108.

Narated By Ibn Abu Mulaika : 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 Apostle.”