Messenger of Allah (ﷺ) said, "I have been commanded (by Allah) to fight people until they testify that there is no true god except Allah, and that Muhammad is the Messenger of Allah, and perform Salat and pay Zakat. If they do so, they will have protection of their blood and property from me except when justified by Islam, and then account is left to Allah".

[Al-Bukhari and Muslim].

Messenger of Allah (ﷺ) said, "He who professes La ilaha illallah (There is no true god except Allah), and denies of everything which the people worship besides Allah, his property and blood become inviolable, and it is for Allah to call him to account".

[Muslim].

I said, "Tell me, O Messenger of Allah, if I meet an infidel, and we fight together, and he cuts off my hands with his sword, then hides from me behind a tree and (then) says he has submitted himself to Allah. Shall I kill him after he has said it?" He (ﷺ) replied, "Do not kill him." I submitted, "But O Messenger of Allah, he cut off one of my hands and only then he said it." Messenger of Allah (ﷺ) then replied, "Do not kill him, for if you do so, he will be in the position in which you were before you killed him (i.e., he will be considered a Muslim and thus his life will be inviolable), and you will be in the position in which he was before he made his testimony (i.e., your life will not be inviolable, for his heirs can ask for Qisas)".

[Al-Bukhari and Muslim].

Messenger of Allah (ﷺ) sent us to Huraqat, a tribe of Juhainah. We attacked that tribe early in the morning and defeated them, (then) a man from the Ansar and I caught hold of a man (of the defeated tribe). When we overcame him, he said: 'La ilaha illallah (There is no true god except Allah).' At that moment, the Ansari spared him, but I attacked him with my spear and killed him. By the time we went back to Al-Madinah, news had already reached Messenger of Allah (ﷺ). He said to me, "O Usamah, did you kill him after he professed La ilaha illallah (There is no true god except Allah)?" I said, "O Messenger of Allah! He professed it only to save his life." Messenger of Allah (ﷺ) repeated, "Did you kill him after he had professed La ilaha illallah?" He went on repeating this to me until I wished I had not embraced Islam before that day (so that I would have not committed this sin).

[Al-Bukhari and Muslim].

Another narration is: Messenger of Allah (ﷺ) said, "Did you kill him in spite of his professing La ilaha illallah?" I said, "O Messenger of Allah! He said out of fear of our arms." He (ﷺ) said, "Why did you not cut his heart open to find out whether he had done so sincerely or not?" He continued repeating it until I wished that I had embraced Islam only that day.

Messenger of Allah (ﷺ) dispatched a contingent of the Muslims to a tribe of the polytheists. The two sides met (in combat) at one place. A man among the polytheists was so dashing that, whenever he intended to kill a man from Muslims, he killed him. Amongst the Muslims, too, was a man looking forward to (an opportunity of) his (polytheist's) unmindfulness. He (the narrator) said: We talked that he was Usamah bin Zaid (May Allah be pleased with him). When he raised his sword, he (the soldier of the polytheists) uttered: "La ilaha illallah (There is no true god except Allah)." But he (Usamah bin Zaid) killed him. When the good news of victory reached Messenger of Allah (ﷺ) he asked him (about the events of the battle), and he informed him about the man (Usamah) and what he had done. He (Messenger of Allah (ﷺ)) sent for him and asked him why he had killed him. He (Usamah) said, "O Messenger of Allah, he struck the Muslims and killed such and such of them." And he named some of them. (He continued): "I attacked him and when he saw the sword he said: 'La ilaha illallah."' Messenger of Allah (ﷺ) said, "Did you kill him?" He (Usamah) replied in the affirmative. The Messenger of Allah then remarked, "What would you do with regard to (the utterance): La ilaha illallah, when it comes (before you) on the Day of Resurrection?" He (Usamah) said, "O Messenger of Allah! Beg forgiveness for me". He (the Prophet (ﷺ)) said, "What would you do with regard to (the utterance): La ilaha illallah, when it comes (before you) on the Day of Resurrection?" He (Messenger of Allah (ﷺ)) added nothing to it but kept repeating, "What would you do with regard to (the utterance): La ilaha illallah, when it comes (before you) on the Day of Resurrection?".

[Muslim].

I heard 'Umar bin Al- Khattab (May Allah be pleased with him) reported saying: "In the lifetime of Messenger of Allah (ﷺ) some people were called to account through Revelation. Now Revelation has discontinued and we shall judge you by your apparent acts. Whoever displays to us good, we shall grant him peace and security, and treat him as a near one. We have nothing to do with his insight. Allah will call him to account for that. But whosoever shows evil to us, we shall not grant him security nor shall we believe him, even if he professed that his intention is good."

[Al-Bukhari].