Al-Matalib al-'Aliyah 2881 — Prohibition of harming anyone who says

It is narrated from Sayyiduna Jundub bin Sufyan al-Bajali (may Allah be pleased with him): I was present with the Prophet (peace and blessings be upon him) when a messenger bringing glad tidings came from a military expedition and told of the unseen help that Allah Almighty had granted to His army, and of the great victory that Allah, the Mighty and Majestic, had decreed for them. He said: "O Messenger of Allah! When Allah Almighty had already utterly defeated the enemy and we were pursuing them, I caught up with a man, sword raised. When he sensed that the sword was about to fall upon him, he turned while running and said: I am a Muslim. But I killed him, although, O Prophet of Allah, he had said it only to seek refuge and save his life." He (peace and blessings be upon him) said: "Why did you not split open his heart, so that you would know whether he was truthful or lying in his claim?" He said: "Even if I had split open his heart, it would not have told me anything; is his heart anything but a piece of flesh?" He (peace and blessings be upon him) said: "You killed him, though you neither knew the secret of his heart nor accepted the words spoken by his tongue." He said: "O Messenger of Allah! Pray for forgiveness for me." He (peace and blessings be upon him) said: "I will never pray for forgiveness for you." Then that man died, and the people buried him, but in the morning he was lying on the surface of the ground; this happened three times. When his people saw this sight of warning, they were deeply ashamed and feared the fate that man had met, so they threw him into one of the mountain ravines.
Imam Abu Ya'la (may Allah have mercy on him) also related a similar narration from Sayyiduna Jundub bin Sufyan al-Bajali (may Allah be pleased with him) through another chain, via Muhammad bin Bakkar.