Chapter 40: Prescribed Punishments (Kitab Al-Hudud)
Narrated Mu' adh ibn Jabal: Abu Musa said: Mu' adh came to me when I was in the Yemen. A man who was Jew embraced Islam and then retreated from Islam. When Mu' adh came, he said: I will not come down from my mount until he is killed. He was then killed. One of them said: He was asked to repent before that.
Sunan Abi Dawud 4355