Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) used to give fatwas on the issue of sarf (currency exchange), until he came to Sayyiduna Umar (may Allah be pleased with him) and asked him, and he disliked it. So Sayyiduna Abdullah (may Allah be pleased with him) retracted that view of his.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8546