Musannaf Ibn Abi Shayba 13:733

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Ibn ‘Awn narrated to us, from Ibn Sirin, who said: A man lent another man five hundred dirhams and stipulated upon him the use of the back of his horse. Ibn Mas’ud said: Whatever he obtained from the back of his horse, it is usury (riba).