Musnad Ahmad 31:22

Isma'il ibn Ibrahim narrated to us, Ayyub narrated to us, from Nafi', who said: Ibn 'Umar said: "Do not sell gold for gold, nor silver for silver, except like for like, and do not increase some of it over others, and do not sell anything absent of it for something present. For I fear for you the *rima'." And *rima'* is usury (*riba*). He said: Then a man narrated to Ibn 'Umar a similar hadith, from Abu Sa'id al-Khudri, narrating it from the Messenger of Allah (ﷺ). He had not finished his statement before he took him to Abu Sa'id, and I was with him. He said: "This man narrated to me a hadith from you, claiming that you narrate it from the Messenger of Allah (ﷺ). Did you hear it?" He said: "My eyes saw it and my ears heard it; I heard the Messenger of Allah (ﷺ) saying: 'Do not sell gold for gold, nor silver for silver, except like for like, and do not increase some of it over others, and do not sell anything absent of it for something present.'"