From Sahih Bukhari (Hadith 3765):
Narrated Ibn Abi Mulaika:Somebody said to Ibn `Abbas, "Can you speak to the chief of the believers Mu`awiyah, as he does not pray except one rak`a as witr?" Ibn `Abbas replied, "He is a Faqih (i.e. a learned man who can give religious verdicts)
Reference: Bukhari 3765