Ibn Abi Mulaykah (may Allah have mercy on him) said that it was said to Ibn Abbas (may Allah be pleased with them both): "What do you think of Mu'awiyah (may Allah be pleased with him)? He prays only one rak'ah of witr." He said: "He did right; he is a man of deep understanding (faqih)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4798