Hadithcore
Sahih al-Bukhari, 3765
sahih

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) ."

حَدَّثَنَا ابْنُ أَبِي مَرْيَمَ، حَدَّثَنَا نَافِعُ بْنُ عُمَرَ، حَدَّثَنِي ابْنُ أَبِي مُلَيْكَةَ، قِيلَ لاِبْنِ عَبَّاسٍ هَلْ لَكَ فِي أَمِيرِ الْمُؤْمِنِينَ مُعَاوِيَةَ، فَإِنَّهُ مَا أَوْتَرَ إِلاَّ بِوَاحِدَةٍ‏.‏ قَالَ إِنَّهُ فَقِيهٌ‏.‏

References2 variants
In-Book Reference
Book 62, Hadith 110
USC-MSA web (English) reference
Vol. 5, Book 57, Hadith 109 (deprecated numbering scheme)