Musannaf Ibn Abi Shayba 13:2046

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Abdullah bin Sa’id al-Maqburi, from his grandfather, from Abu Hurairah, that the Prophet (peace and blessings of Allah be upon him) said: "Usury (Riba) is seventy sins, the easiest of which is a man marrying his mother. And the most usurious of usury is a man violating the honor of his brother."