Musnad Ahmad 941:75

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Amr ibn Dinar, from Dhakwan, who said: Abu Sa‘id al-Khudri sent me to Ibn ‘Abbas. He said: Say to him regarding *al-sarf* (currency exchange): Did you hear from the Messenger of Allah, may Allah bless him and grant him peace, what we did not hear? Or did you read in the Book of Allah what we did not read? He said: By no means do I say that. But I heard Usamah ibn Zayd narrating that the Messenger of Allah, may Allah bless him and grant him peace, said: "There is no *riba* (usury) except in *al-dayn* (debt)," or he said: "In *al-nasi'ah* (deferred payment)."