Musnad Abu Ya'la al-Mawsili 155:267

Abu Ma'mar narrated to us, Ibn Fudayl narrated to us, from his father, from Abu Zur'ah, from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) said: "Dates for dates, wheat for wheat, and salt for salt, like for like. Whoever gives more or takes more, then it is usury, except for what differs in its colors."