It is narrated from Sayyiduna Usamah bin Zayd (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) sent us in an expedition to al-Huraqat. After the initial attack, they scattered in flight. We caught a man, and he began saying 'La ilaha illallah' (There is no god worthy of worship except Allah), but we killed him. This troubled my heart. I mentioned it to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "What answer will you give for 'La ilaha illallah' on the Day of Resurrection?" I said: O Messenger of Allah! He only said it out of fear of the weapons and of being killed. He (peace and blessings be upon him) said: "Did you split open his chest to see why he said the kalimah? O Usamah! What answer will you give for this 'La ilaha illallah' on the Day of Resurrection?" He said: He (peace and blessings be upon him) kept repeating this until I began to think: If only I had become a Muslim only today! Abu Zabyan said that Sayyiduna Sa'd (may Allah be pleased with him) said: By Allah, I will not kill (a Muslim) until Dhul-Butayn, meaning Usamah (may Allah be pleased with them both), kills him. A man said: Has Allah Almighty not said: "And fight them until there is no more fitnah" [Surah al-Baqarah: 193]? Sayyiduna Sa'd (may Allah be pleased with him) said: We fought until the fitnah was ended; do you and your companions want us to fight until fitnah arises?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15847