Musnad Ahmad 941:9

Sufyan ibn 'Uyaynah narrated to us, 'Amr—meaning Ibn Dinar—narrated to us, from Abu Salih, who said: I heard Abu Sa'id saying: "Gold for gold, weight for weight." He said: So I met Ibn 'Abbas and said: "Do you see what you are saying? Is it something you found in the Book of Allah or heard from the Messenger of Allah, may Allah bless him and grant him peace?" He said: "It is not something I found in the Book of Allah or heard from the Messenger of Allah, may Allah bless him and grant him peace. However, Usamah ibn Zayd informed me that the Messenger of Allah, may Allah bless him and grant him peace, said: 'Riba (usury) is in the deferred payment (an-nasi'ah).'"