Ibn Abi Mulaykah (may Allah have mercy on him) said that Mu'awiyah (may Allah be pleased with him) prayed one rak'ah of witr after Isha, and a freed slave of Ibn Abbas (may Allah be pleased with them both) was with him. He told Ibn Abbas (may Allah be pleased with them both), and he said: "Leave him, for he is a Companion of the Prophet (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4797