Narrated Usama bin Zaid:
Allah's Apostle sent us towards Al-Huruqa, and in the morning we attacked them and defeated them. I and an Ansari man followed a man from among them and when we took him over, he said, "La ilaha illal-Lah." On hearing that, the Ansari man stopped, but I killed him by stabbing him with my spear.
When we returned, the Prophet came to know about that and he said, "O Usama! Did you kill him after he had said "La ilaha ilal-Lah?"
I said, "But he said so only to save himself."
The Prophet kept on repeating that so often that I wished I had not embraced Islam before that day.