Sayyiduna Anas bin Malik (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Cheating a person who places his trust (in you) is usury (riba)."
(b) Sayyiduna Ali (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "Cheating a person who places his trust (in you) is usury."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10925