From Sahih Bukhari (Hadith 2060):
Narrated Abu Al-Minhal:I used to practice money exchange, and I asked Zaid bin 'Arqam about it, and he narrated what the Prophet said in the following: Abu Al-Minhal said, "I asked Al-Bara' bin `Azib and Zaid bin Arqam about practicing money exchange. They replied, 'We were traders in the time of Allah's Messenger (ﷺ) and I asked Allah's Messenger (ﷺ) about money exchange. He replied, 'If it is from hand to hand, there is no harm in it; otherwise it is not permissible
Reference: Bukhari 2060