Musannaf Ibn Abi Shayba 13:2875

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Jabir, from al-Sha’bi and al-Hakam, regarding a man who buys food and there is an excess: They both said: If it was a mistake, he returns it, and if it was an increase, he returns it.