I asked Fadala bin Ubayd about hanging the severed hand around the neck, and he said, "That is the sunnah. The Messenger of Allah ﷺ cut off a man's hand and then hung it around his neck."
حدثنا أبو بكر بن أبي شيبة وأبو بشر بكر بن خلف ومحمد بن بشار وأبو سلمة الجوباري يحيى بن خلف قالوا حدثنا عمر بن علي بن عطاء بن مقدم عن حجاج عن مكحول
سألت فضالة بن عبيد عن تعليق اليد في العنق فقال السنة قطع رسول الله ﷺ يد رجل ثم علقها في عنقه
[Its chain of transmission is daif] Abu Dawud narrated it in the chapter on Punishments, section on "the thief whose hand is hung around his neck," no. 4411, from the hadith of Umar bin Ali, and Tirmidhi graded it hasan, no. 1447. Nasai said, no. 4986, 8/92, "Hajjaj bin Artah is daif and his hadith is not to be relied upon." See also nos. 496 and 1129.