Musnad Ahmad 31:131

Bakr ibn Isa narrated to us, Jami ibn Matar al-Habti narrated to us, Abu Rawbah Shaddad ibn Imran al-Qaysi narrated to us, from Abu Sa’id al-Khudri, that Abu Bakr came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "O Messenger of Allah, I passed by such-and-such a valley, and there was a man, humble in appearance and of good demeanor, praying." The Prophet, may Allah bless him and grant him peace, said to him: "Go to him and kill him." He said: So Abu Bakr went to him, but when he saw him in that state, he disliked killing him, so he returned to the Messenger of Allah, may Allah bless him and grant him peace. He said: The Prophet, may Allah bless him and grant him peace, said to Umar: "Go and kill him." So Umar went and saw him in the same state that Abu Bakr had seen him in. He said: So he disliked killing him. He said: He returned and said: "O Messenger of Allah, I saw him praying humbly, so I disliked killing him." He said: "O Ali, go and kill him." He said: So Ali went, but he did not see him. Ali returned and said: "O Messenger of Allah, he did not see him." He said: The Prophet, may Allah bless him and grant him peace, said: "Indeed, this man and his companions recite the Quran, but it does not pass beyond their throats. They pass through the religion just as an arrow passes through the game, then they will not return to it until the arrow returns to its notch. Kill them, for they are the worst of creation."