(12) باب الْحَاكِمِ يَحْكُمُ بِالْقَتْلِ عَلَى مَنْ وَجَبَ عَلَيْهِ دُونَ الإِمَامِ الَّذِي فَوْقَهُ
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.
Reference: Sahih al-Bukhari 7157
In-book reference: Book 93, Hadith 21
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 89, Hadith 271