Musnad Ahmad Hadith 21802

It is narrated from Usama bin Zaid (may Allah be pleased with him) that once the Noble Prophet (ﷺ) sent us towards a tribe in the sandy areas of Juhaina. We attacked them in the morning and began fighting. Among them was a man who, whenever he appeared before us, fought with the greatest bravery, and when his people turned their backs and fled, he would protect them from behind. I, along with an Ansari, surrounded him. As soon as we tightened the circle around him, he immediately said, "La ilaha illallah." At this, the Ansari withdrew his hand, but I killed him. When the Noble Prophet (ﷺ) learned of this, he said, "Usama! After he had said 'La ilaha illallah,' you still killed him?" I submitted, "O Messenger of Allah! He said this to save his life." The Noble Prophet (ﷺ) repeated his statement so many times that I began to wish that I had accepted Islam only that day.