Musannaf Ibn Abi Shayba 13:462

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Usama bin Zayd, he said: I heard Sa’id bin al-Musayyib, and he was asked about a man who bought a camel, then the buyer regretted it and wanted to return it and return eight dirhams along with it. Sa’id said: "There is no harm in it. Verily, usury (riba) is only in that which is measured and weighed, from that which is eaten and drunk."