Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from Sayyiduna Umar (may Allah be pleased with him) concerning the sale of gold — and he had not heard anything about it from the Prophet (peace and blessings be upon him) — that Sayyiduna Umar (may Allah be pleased with him) said: Do not sell gold for gold or silver for silver except equal for equal, and do not give one more than the other; I fear usury (riba) for you. The narrator said: A man of the Ansar narrated a hadith from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him). Nafi' said: Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) took the Ansari by the hand, and I was with the two of them, and we came to Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him). He said: O Abu Sa'id! This man has narrated (such-and-such) a hadith from you; what is it? He mentioned it, and Sayyiduna Abu Sa'id (may Allah be pleased with him) said: Yes, my ears heard it and my eyes saw it — he repeated this three times. He pointed with his fingers level with his eyes, (saying) that he (peace and blessings be upon him) said: "Do not sell gold for gold or silver for silver except equal for equal; do not sell anything (of it) that is on credit for what is in cash; and do not give one more than the other."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10490