It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever intercedes for his brother in some matter, and is then given a gift for it and accepts it, has stepped into one of the great doors of usury (riba)."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7822