Sunan an-Nasai 19314
In-book no 4578أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ حُمَيْدِ بْنِ قَيْسٍ الْمَكِّيِّ، عَنْ مُجَاهِدٍ، قَالَ قَالَ ابْنُ عُمَرَ الدِّينَارُ بِالدِّينَارِ وَالدِّرْهَمُ بِالدِّرْهَمِ لاَ فَضْلَ بَيْنَهُمَا هَذَا عَهْدُ نَبِيِّنَا صلى الله عليه وسلم إِلَيْنَا .
It was narrated that Mujahid said: "Umar said: Dinar for Dinar, Dirham for Dirham, no difference between them, this is the obligation that our Prophet enjoined upon us."