Musannaf Ibn Abi Shayba 13:1957

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Wahb ibn ‘Uqbah, he said: I asked al-Sha’bi about a people who used to buy the milk of cows for known days, then they would buy it [again]. He said: It is not permissible except hand-to-hand.