Musannaf Ibn Abi Shayba 13:2325

Abu Bakr narrated to us, he said: Ibn Abi Za'idah narrated to us, from Shu'bah, who said: I asked al-Hakam and Hammad about a man who passed by a tax collector (ashir) with slaves, and he said: "These are free people." Al-Hakam said: "It is nothing (i.e., it has no legal consequence)." And Hammad said: "I fear that they might become free."