Muawiya offered one rakʿa witr prayer after the ʿIsha prayer, and at that time a freed slave of Ibn ʿAbbas was present. He (i.e. the slave) went to Ibn ʿAbbas (and told him that Muawiya offered one rakʿa witr prayer). Ibn ʿAbbas said, "Leave him, for he was in the company of Messenger of Allah ﷺ."  

دَعْهُ فَإِنَّهُ صَحِبَ رَسُولَ اللَّهِ ﷺ  

Somebody said to Ibn ʿAbbas, "Can you speak to the chief of the believers Muʿawiyah, as he does not pray except one rakʿa as witr?" Ibn ʿAbbas replied, "He is a Faqih (i.e. a learned man who can give religious verdicts) ."  

إِنَّهُ فَقِيهٌ  

Muawiya said (to the people), "You offer a prayer which we, who were the companions of the Prophet ﷺ never saw the Prophet ﷺ offering, and he forbade its offering," i.e. the two rakʿat after the compulsory ʿAsr prayer.  

إِنَّكُمْ لَتُصَلُّونَ صَلاَةً لَقَدْ صَحِبْنَا النَّبِيَّ ﷺ فَمَا رَأَيْنَاهُ يُصَلِّيهَا وَلَقَدْ نَهَى عَنْهُمَا يَعْنِي الرَّكْعَتَيْنِ بَعْدَ الْعَصْرِ