Musnad Ahmad 30:2511

Yahya narrated to us, he said: Fudayl ibn Ghazwan narrated to us, he said: Ibn Abi Nu'm narrated to me, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, who said: "Gold for gold, and silver for silver, and paper [currency] for paper [currency], like for like, hand to hand. Whoever increases or seeks an increase, then he has committed usury."