It is narrated from Sayyiduna Abdullah bin Salam (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "A single dirham of usury (riba) that a man acquires is a greater sin in the sight of Allah than committing fornication thirty-three times in Islam."
It is also narrated from the Messenger of Allah (peace and blessings be upon him) that he said: "Usury has seventy-two doors; the lightest of them (in sin) is like a man who commits fornication with his own mother in Islam."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14008