Riyad us-Salihin Hadith 394

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].