Sahih Bukhari
Companions of the Prophet
Volume: 005 / Book Number: 057 / Hadith Number: 108
Narrated 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."