Abu Dawud Book 8, Hadith Number 2646.

Narated By Furat ibn Hayyan : The Apostle of Allah (PBUH) commanded to kill him: he was a spy of AbuSufyan and an ally of a man of the Ansar. He passed a circle of the Ansar and said: I am a Muslim. A man from the Ansar said, Apostle of Allah, he is saying that he is a Muslim. The Apostle of Allah (PBUH) said: There are people among you in whose faith we trust. Furat ibn Hayyan is one of them.