Musannaf Ibn Abi Shayba 13:1584

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hasan ibn Salih, from Mujalid, from al-Sha’bi, from Shurayh, who said: "It is for his master." And Sufyan narrated from Mughirah, from Ibrahim: "He places it in the necks [i.e., for the freeing of slaves]."