Jundab bin Abdullah reported,

During the time of the turmoil involving Ibn Zubayr, he sent word to Asas bin Salama, saying, "Gather for me a group of your brothers so I may speak to them." So Asas sent a messenger to them.When they had assembled, Jundab arrived wearing a yellow hooded cloak and said, "Carry on with what you were discussing." The conversation went around the circle, and when it came to his turn, he pulled the hood back from his head and said, "I have come to you intending to tell you something about your Prophet."The Messenger of Allah ﷺ dispatched an expedition of Muslims against a group of polytheists. The two sides met in battle, and there was a man among the polytheists who, whenever he wished to single out a Muslim and kill him, would do so. Then a man from the Muslims watched for a moment when he was off guard. We used to say that this man was Usama bin Zayd. When he raised his sword over him, the polytheist said, "There is no god but Allah." But he killed him anyway.The bearer of news came to the Prophet ﷺ, who questioned him, and he reported what had happened, including the account of how that man had acted. So the Prophet ﷺ summoned him and asked, "Why did you kill him?"He replied, "Messenger of Allah, he had inflicted heavy losses on the Muslims and had killed so-and-so and so-and-so," naming several men, "so I charged at him. When he saw the sword, he said, 'There is no god but Allah.'"The Messenger of Allah ﷺ said, "And you killed him?"He answered, "Yes."The Prophet ﷺ said, "What will you do about 'There is no god but Allah' when it comes on the Day of Resurrection?"He said, "Messenger of Allah, ask forgiveness for me."The Prophet ﷺ said again, "What will you do about 'There is no god but Allah' when it comes on the Day of Resurrection?"And he kept repeating nothing more than, "What will you do about 'There is no god but Allah' when it comes on the Day of Resurrection?"

عن جندب بن عبد الله البجلي

أنه بعث إلى عسعس بن سلامة زمن فتنة ابن الزبير، فقال: اجمع لي نفرا من إخوانك حتى أحدثهم فبعث رسولا إليهم. فلما اجتمعوا جاء جندب وعليه برنس أصفر فقال: تحدثوا بما كنتم تحدثون به حتى دار الحديث. فلما دار الحديث إليه، حسر البرنس عن رأسه فقال: إني أتيتكم ولا أريد أن أخبركم عن نبيكم، إن رسول الله ﷺ بعث بعثا من المسلمين إلى قوم من المشركين وأنهم التقوا فكان رجل من المشركين إذا شاء أن يقصد إلى رجل من المسلمين قصد له فقتله، وإن رجلا من المسلمين قصد غفلته- قال: وكنا نحدث أنه أسامة بن زيد- فلما رفع عليه السيف قال: لا إله إلا الله فقتله! فجاء البشير إلى النبي ﷺ فسأله فأخبره حتى أخبره خبر الرجل كيف صنع فدعاه فسأله فقال: لم قتلته؟ قال: يا رسول الله أوجع في المسلمين، وقتل فلانا وفلانا وسمى له نفرا، وإني حملت عليه. فلما رأى السيف قال: لا إله إلا الله! قال رسول الله ﷺ: أقتلته؟ 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؟ قال: فجعل لا يزيده على أن يقول: كيف تصنع بلا إله إلا الله إذا جاءت يوم القيامة؟