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

English translation · Sahih al-Bukhari