It is narrated from Sayyiduna Usama bin Zaid (may Allah be pleased with him), he says: The Messenger of Allah (peace and blessings be upon him) sent us towards the tribe of Harqa from Banu Juhaina. We reached them early in the morning and fought with them. Among them was a man who, whenever his people came forward to fight, he fought bravely, and when they turned their backs and fled, he defended them. I and an Ansari overpowered him. When we had subdued him, he recited "La ilaha illallah" (There is no god but Allah). The Ansari (may Allah be pleased with him) withheld his hand from killing him, but I killed him. When this matter reached the Noble Prophet (peace and blessings be upon him), he said: O Usama! Did you kill him even after he had recited the Kalimah? I said: O Messenger of Allah! He did not recite the Kalimah from his heart; he only recited it to save his life. But (to refute me) the Prophet (peace and blessings be upon him) repeated his statement so much that I wished I had not accepted Islam before this, but had become a Muslim today (so that this mistake would not have occurred from me, or it would have been forgiven due to the acceptance of Islam).

(Second Chain) Sayyiduna Usama (may Allah be pleased with him) says: I submitted, "O Messenger of Allah! He only recited the kalimah to save himself from blame or being killed." The Prophet (peace and blessings be upon him) said, "Did you cut open his heart to see whether he recited the kalimah for that reason or for some other reason? Who will be with you on the Day of Judgment to face LAA ILAAHA ILLALLAH?" The Prophet (peace and blessings be upon him) kept repeating this statement to such an extent that I wished I had only accepted Islam that day (so that this mistake would not have occurred from me and I would have been saved from the Prophet's reproach).