Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "Usury (riba) has seventy-two doors, the least of which is that a man should have relations with his own mother; and the greatest usury is that a man assails the honour of his brother."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Umar bin Rashid, whom al-Ijli declared thiqah (trustworthy), while the majority of the Imams declared him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب البيوع / 6575