"The Messenger of Allah forbade selling silver for silver and gold for gold, unless it was of equal amounts. And he told us to sell gold for silver however we wanted, and silver for gold however we wanted.
Sunan an-Nasa'i Book 44, Hadith 130
← Previous Hadith "I asked Al-Bara bin 'Azib about money exchange. He said: 'Ask Zaid bin Arqam, for he is better than me and more knowledgeable.' So I asked Zaid and he said: 'Ask Al-Bara for he is better than me and more knowledgeable.' And they both said: 'The Messenger of Allah forbade (selling) silver for gold on credit."'

Sunan an-Nasa'i Book 44, Hadith 129 Next Hadith → "The Messenger of Allah forbade us from selling silver for silver unless it was of equal amounts, or selling gold for gold unless it was of equal amounts. The messenger of Allah said: 'Sell gold for silver however you want, and silver for gold however you want."'

Sunan an-Nasa'i Book 44, Hadith 131