Ibn Umar (may Allah be pleased with them both) said: "A dinar for a dinar and a dirham for a dirham; there is no excess (tafadul) between them." This was the covenant of our Prophet (peace and blessings be upon him) to us, and this is our covenant to you.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10492