Prohibition of harming anyone who says: There is none worthy of worship except Allah

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.

It is narrated from Harun bin Ri'ab that the Messenger of Allah (peace and blessings be upon him) sent out an army, and they were granted victory. Then they sent a bearer of glad tidings to the Messenger of Allah (peace and blessings be upon him). While he was telling him (peace and blessings be upon him) of the victory granted by Allah Almighty and the number of those slain whom Allah had destroyed, he said: "I got hold of one of their men on my own, and when I overpowered him to kill him, he cried out: I am a Muslim. But I killed him." On hearing this, the Messenger of Allah (peace and blessings be upon him) said: "Did you kill him when he had already said: I am a Muslim?" He said: "O Messenger of Allah! He said that only to seek refuge, out of fear of death." He (peace and blessings be upon him) said: "Then did you split open his heart and look?" He said: "O Messenger of Allah! Could I have known that by splitting open his heart?" He (peace and blessings be upon him) said: "You neither accepted what his tongue said nor came to know the state of his heart; you are indeed his killer. Go away from me and do not keep my company." The narrator said: Then that man died, and the earth threw him out of the grave twice, so he was cast into one of the mountain valleys.
Some of the people of knowledge said: The earth conceals people even more foul and wicked than him, but this incident was made manifest only as a lesson and admonition for people.

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you points his spear at a (hostile) man, and it has come right up to the hollow of his throat (his jugular), and then that man says 'La ilaha illallah' (there is no god worthy of worship except Allah), then (the attacker) should immediately lift his spear away from him." The narrator said that Abu Ubaydah (may Allah have mercy on him) said: "So Allah, the Blessed and Exalted, has made this word (of tawhid) a source of safety for the Muslim and a means of protecting and safeguarding his blood, and (likewise) He has made the jizyah a source of safety for the (protected) disbeliever and a means of safeguarding his life."
Takhrij and grading: This hadith is narrated in Musnad Ahmad (2883); its chain is broken, because the hearing of Abu Ubaydah bin Abdullah bin Mas'ud from his father is not established; however, its meaning regarding the sanctity of the blood of one who professes the word of tawhid is established and supported by other sahih and authentic hadith.