Abu Rafi' (may Allah be pleased with him) said: Umar bin al-Khattab (may Allah be pleased with him) used to sit with me and teach me a verse. I would forget it, and I would call out to him: O Commander of the Believers! I have forgotten it. He would come back and teach it to me. I said to him: I fashion gold into jewellery and I sell it at its weight. Should I take my wage?
He said: Sell gold for gold only equal in weight, and silver for silver equal in weight. Do not take anything extra.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10549