Musannaf Ibn Abi Shayba 13:2552

Abu Bakr narrated to us, he said: Ali bin Mushir narrated to us, from al-Shaybani, from Uqbah Abu al-Akhdar, who said: Ibn Umar was asked about gold being sold on credit. He said: I heard Umar bin al-Khattab upon this pulpit, and he was asked about it, so he said: "Every moment that he delays it, it is usury."