Abu Dawud Book 2, Hadith Number 1244.

Narated By ‘Abd Allah b. Unais : The Apostle of Allah (PBUH) send me to Khalid b. Sufyan al-Hudhali. This was towards ‘Uranah and ‘Arafat. He (the Prophet) said: Go and kill him. I saw him when the time of the afternoon prayer had come. I said: I am afraid if a fight takes place between me and him (Khalid b. Sufyan), that might delay the prayer. I proceeded walking towards him while I was praying making a sign. When I reached near him, he said to me: Who are you? I replied: A man from the Arabs; it came to me that you were gathering (an army) for this man (i.e. the Prophet). Hence I came to you in connection with this matter. He said: I am (engaged) in this (work). I then walked along with him for a while.; when it became convenient for me, I dominated him with my sword until he became cold (dead).