Musnad Ahmad 830:59

Rawh narrated to us, Uthman al-Shahham narrated to us, Muslim ibn Abi Bakrah narrated to us, from his father, that the Prophet of Allah (peace and blessings of Allah be upon him) passed by a man who was prostrating while he (the Prophet) was heading to prayer. He finished the prayer and returned to him while he was still prostrating. The Prophet (peace and blessings of Allah be upon him) stood and said, "Who will kill this man?" A man stood up, bared his arms, drew his sword, and shook it. Then he said, "O Prophet of Allah, may my father and mother be sacrificed for you, how can I kill a prostrating man who testifies that there is no god but Allah and that Muhammad is His servant and His Messenger?" Then he said, "Who will kill this man?" Another man stood up and said, "I will." He bared his arms, drew his sword, and shook it until his hand trembled. He said, "O Prophet of Allah, how can I kill a prostrating man who testifies that there is no god but Allah and that Muhammad is His servant and His Messenger?" The Prophet (peace and blessings of Allah be upon him) then said, "By the One in Whose hand is the soul of Muhammad, if you had killed him, it would have been the first of the tribulation and the last of it."