Sayyiduna Usamah bin Zayd (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) sent us to al-Huraqat, and those people became frightened and alarmed. We caught a man and he said "La ilaha illallah" (There is no god but Allah), but we killed him. Then something troubled me within myself, so I mentioned it to the Prophet (peace and blessings be upon him), and he said: "O Usamah! Who will save you from 'La ilaha illallah' on the Day of Resurrection?" I said: O Messenger of Allah, he only said it out of fear of the weapons. He said: "Did you split open his heart (to see)? Who will save you from this very 'La ilaha illallah'?" He (peace and blessings be upon him) kept on saying this until I wished that I had become a Muslim only that day.
Abu Zabyan (may Allah have mercy on him) said: Sayyiduna Sa'd (may Allah be pleased with him) said: By Allah, I will not kill (anyone who says La ilaha illallah) until Dhu al-Butayn, meaning Usamah, kills him. A man said: Has Allah not said: "And fight them until there is no fitnah" [Surah al-Baqarah: 193]? Sayyiduna Sa'd (may Allah be pleased with him) said: We fought until the fitnah ended, and no fitnah remained; but you and your companions want there to be fitnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16804