Abu Sa'id al-Khudri (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: "Do not sell gold for gold unless it is equal on both sides; do not allow any deficiency or excess on either side. Do not sell silver for silver unless it is equal on both sides; do not allow any deficiency or excess on either side. And do not sell on credit for cash."