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."

حَدَّثَنَا الْحَسَنُ بْنُ بِشْرٍ، حَدَّثَنَا الْمُعَافَى، عَنْ عُثْمَانَ بْنِ الأَسْوَدِ، عَنِ ابْنِ أَبِي مُلَيْكَةَ، قَالَ أَوْتَرَ مُعَاوِيَةُ بَعْدَ الْعِشَاءِ بِرَكْعَةٍ وَعِنْدَهُ مَوْلًى لاِبْنِ عَبَّاسٍ، فَأَتَى ابْنَ عَبَّاسٍ فَقَالَ دَعْهُ، فَإِنَّهُ صَحِبَ رَسُولَ اللَّهِ ﷺ.