Musannaf Ibn Abi Shayba 13:3341

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, who said: I asked al-Sha'bi and al-Hakam about a man who rents camels, then rents them out for more than what he rented them for. They said: There is no harm in it if he works on them himself or hires a worker for them.