Compiled by Imam Yahya ibn Sharaf al-Nawawi
وعن جندب بن عبد الله، رضي الله عنه ، أن رسول الله صلى الله عليه وسلم، بعث بعثاً من المسلمين إلى قوم من المشركين، وأنهم التقوا فكان رجلاً من المشركين إذا شاء أن يقصد إلى رجل من المسلمين قصد له فقتله، وأن رجلاً من المسلمين قصد غفلته، وكنا نتحدث أنه أسامة بن زيد، فلما رفع عليه السيف ، قال: لا إله إلا الله، فقتله، فجاء البشير إلى رسول الله ، صلى الله عليه وسلم فسأله ، وأخبره، حتى أخبره خبر الرجل كيف صنع ، فدعاه فسأله، فقال: "لم قتلته؟ فقال: يا رسول الله أوجع في المسلمين، وقتل فلانا وفلانا -وسمى له نفراً- وإني حملت عليه، فلما رأى السيف قال: لا إله إلا الله. قال رسول الله صلى الله عليه وسلم: "اقتلته؟" قال نعم : قال: "فكيف تصنع بلا إله إلا الله، إذا جاءت يوم القيامة؟ قال: يا رسول الله استغفر لي. قال: "وكيف تصنع بلا إله إلا الله إذا جاءت يوم القيامة؟ " فجعل لا يزيد على أن يقول: " كيف تصنع بلا إله إلا الله إذا جاءت يوم القيامة" ((رواه مسلم)).
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].