It is narrated from Mujahid that Ibn Umar (may Allah be pleased with him) said: Dinar is for dinar and dirham is for dirham; any increase in them is not permissible. This is the covenant of our Prophet (ﷺ) with us and our covenant with you.
Hadith Referenceمسند الإمام الشافعي / كتاب البيوع / 1389
Hadith Takhrijصحيح : أخرجه النسائي البيوع، باب بيع الدرهم بالدرهم (4572) والبيهقي: 5 / 279- وعبدالرزاق (14574)۔