Abu Dawud Book 33, Hadith Number 4341.

Narated By Mu’adh ibn Jabal : AbuMusa 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.