Sayyiduna Sa'id bin Zayd (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The greatest usury (riba) is to violate the honour of a Muslim brother unjustly."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21127