It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that once I asked the Prophet (peace and blessings be upon him) about this issue: Can I buy gold in exchange for silver or silver in exchange for gold? The Prophet (peace and blessings be upon him) said: "When you receive one of these two in exchange for the other, do not separate from your companion until there remains no transaction between you and him."