Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates: Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) came to the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah, I passed through such-and-such a valley, and there was a humble man of good appearance there, praying." The Prophet (peace and blessings be upon him) said: "Go and kill him." The narrator says: Sayyiduna Abu Bakr (may Allah be pleased with him) went to that man, but when he saw him in that state he disliked killing him, and he returned to the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) said to Sayyiduna Umar (may Allah be pleased with him): "Go and kill him." Sayyiduna Umar (may Allah be pleased with him) went and saw him in the same state in which Sayyiduna Abu Bakr (may Allah be pleased with him) had seen him. The narrator says: He too came back and said: "O Messenger of Allah, I saw him praying with humility, so I disliked killing him." The Prophet (peace and blessings be upon him) said: "O Ali, go and kill him." Sayyiduna Ali (may Allah be pleased with him) went but did not see the man, so Sayyiduna Ali (may Allah be pleased with him) came back and said: "O Messenger of Allah, I did not see him." The narrator says: The Prophet (peace and blessings be upon him) said: "This man and his companions will recite the Qur'an, but it will not go beyond their throats. They will pass out of the religion as an arrow passes through the target, and they will not return to the religion until the arrow returns to its place on the bowstring. Kill them, for they will be the worst of creation."
Ahmad recorded it, and its narrators are thiqat (trustworthy).