"Whoever has intercourse with an animal, there is no prescribed punishment upon him." Muhammad bin Bashshar narrated this to us, saying Abdul-Rahman bin Mahdi narrated to us from Sufyan Thawri.This is more authentic than the first hadith, and this is what is acted upon according to the people of knowledge. It is the view of Ahmad and Ishaq.
وقد روى سفيان الثوري عن عاصم عن أبي رزين
من أتى بهيمة فلا حد عليه حدثنا بذلك محمد بن بشار حدثنا عبد الرحمن بن مهدي حدثنا سفيان الثوري وهذا أصح من الحديث الأول والعمل على هذا عند أهل العلم وهو قول أحمد وإسحق
Sourcing, [its chain of transmission is hasan]. Abu Dawud also narrated it in the Book of Punishments, chapter on one who has intercourse with an animal, hadith: 4464, from the hadith of Abdul-Aziz bin Muhammad, with the same wording. See Nayl al-Maqsud, part 2, p: 963, the report of Ibn Abbas, Abu Dawud, hadith: 4465, and Nasai in al-Kubra, hadith: 7341, from Aasim with the same wording. Nasai said, this is not sahih, and this report concerns one who is not married, whereas one who is married, his punishment is execution, as stated in the marfu hadith.