Musannaf Ibn Abi Shayba 13:3222

Abu Bakr narrated to us, he said: Ishaq bin Mansur narrated to us, he said: Hureym narrated to us, from Abu Ishaq, from Abdullah bin Shaddad, that al-Hasan bin Ali passed by a shepherd who was grazing. He (the shepherd) came to him with a sheep and gifted it to him. He (al-Hasan) said to him, "Are you a free man or a slave?" He said, "A slave." So he returned it to him. He (the shepherd) said, "It is mine." So he accepted it from him. Then he bought him and bought the sheep, and he emancipated him and gave the sheep to him.