Musnad Ahmad 712:1

'Ali ibn 'Abdullah narrated to us, Bishr ibn al-Sari narrated to us. 'Abdullah ibn Ahmad said: And Abu Khaythama narrated to me, Bishr ibn al-Sari narrated to us, Sufyan narrated to us, from Abu Ishaq, from Haritha ibn Mudarrib, from Furat ibn Hayyan, that the Prophet, peace and blessings of Allah be upon him, ordered his killing, and he was a spy for Abu Sufyan and an ally. He passed by a circle of the Ansar and said: "I am a Muslim." They said: "O Messenger of Allah, he claims that he is a Muslim." He said: "Indeed, among you are men whom we entrust to their faith, among them is Furat ibn Hayyan."