Musannaf Ibn Abi Shayba 13:2326

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, regarding a man who passed by a tax collector with a slave and said, "He is free." He said: He did not consider that he would be emancipated by this statement, and he did not see any harm in saying it.