Abu Dawud Book 33, Hadith Number 4350.

Narated By AbuBakr : AbuBarzah said: I was with AbuBakr. He became angry at a man and uttered hot words. I said: Do you permit me, Caliph of the Apostle of Allah (PBUH), that I cut off his neck? These words of mine removed his anger; he stood and went in. He then sent for me and said: What did you say just now? I said: (I had said:) Permit me that I cut off his neck. He said: Would you do it if I ordered you? I said: Yes. He said: No, I swear by Allah, this is not allowed for any man after Muhammad (PBUH).