I asked Al-Bara' bin `Azib and Zaid bin Arqam about money exchanges. Each of them said, "This is better than I," and both of them said, "Allah's Messenger (ﷺ) forbade the selling of silver for gold on credit. "
Sahih al-Bukhari Book 34, Hadith 129
← Previous Hadith I heard Abu Sa`id Al-Khudri saying, "The selling of a Dinar for a Dinar, and a Dirham for a Dirham (is permissible)." I said to him, "Ibn `Abbas does not say the same." Abu Sa`id replied, "I asked Ibn `Abbas whether he had heard it from the Prophet (ﷺ) s or seen it in the Holy Book. Ibn `Abbas replied, "I do not claim that, and you know Allah's Messenger (ﷺ) better than I, but Usama informed me that the Prophet had said, 'There is no Riba (in money exchange) except when it is not done from hand to hand (i.e. when there is delay in payment).' "

Sahih al-Bukhari Book 34, Hadith 128 Next Hadith → that his father said, "The Prophet (ﷺ) forbade the selling of gold for gold and silver for silver except if they are equivalent in weight, and allowed us to sell gold for silver and vice versa as we wished."

Sahih al-Bukhari Book 34, Hadith 130