Musannaf Ibn Abi Shayba 18:683

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Isma'il ibn Salim, from al-Sha'bi, he said: I heard him say: If a man carries a boy who has not reached puberty on his mount, and something happens to him [the boy], then it is upon the one who carried him. If he [the boy] has reached puberty and something happens to him, then he is responsible. And regarding the slave, it is the same as that.