Sahih Bukhari Volume 009, Book 083, Hadith Number 011.
Narrated By Usama bin Zaid bin Haritha : Allah's Apostle sent us (to fight) against Al-Huraqa (one of the sub-tribes) of Juhaina. We reached those people in the morning and defeated them. A man from the Ansar and I chased one of their men and when we attacked him, he said, "None has the right to be worshipped but Allah." The Ansari refrained from killing him but I stabbed him with my spear till I killed him. When we reached (Medina), this news reached the Prophet. He said to me, "O Usama! You killed him after he had said, 'None has the right to be worshipped but Allah?"' I said, "O Allah's Apostle! He said so in order to save himself." The Prophet said, "You killed him after he had said, 'None has the right to be worshipped but Allah." The Prophet kept on repeating that statement till I wished I had not been a Muslim before that day.