Abu al-Jawza' (may Allah have mercy on him) said: I served Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) for nine years. Once a man came and asked about (taking) two dirhams for one dirham. Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) cried out and said: This man is telling me to feed him usury! The people around him said: But we used to do this on the basis of your fatwa. Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: I used to give that fatwa until Sayyiduna Abu Sa'id and Sayyiduna Abdullah bin Umar (may Allah be pleased with them) narrated to me from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) "had forbidden it"; therefore I too forbid you from it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10500