Musannaf Ibn Abi Shayba 33:897

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: Thabit informed us that Mutarrif ibn 'Abd Allah said: "If I had two souls, I would send one of them ahead of the other. If it encountered good, the other would follow it, and if not, I would restrain them both. But it is only one soul; I do not know what it will encounter: good or evil."