Sahih Bukhari

Narrated Abu Musa:A man embraced Islam and then reverted back to Judaism. Mu`adh bin Jabal came and saw the man with Abu Musa. Mu`adh asked, ""What is wrong with this (man)?"" Abu Musa replied, ""He embraced Islam and then reverted back to Judaism."" Mu`adh said, ""I will not sit down unless you kill him (as it is) the verdict of Allah and His Apostle