Musannaf Ibn Abi Shayba 13:1249

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Dawud ibn Abi Hind, from Sa’id ibn Jubayr, he said: I saw Ibn Umar, when he owed silver (dirhams), he would give its value in dinars if it was established at a [specific] rate. And when he owed dinars, he would give silver (dirhams) at their value.