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."
Sahih al-Bukhari Book 34, Hadith 14
← Previous Hadith The Prophet (ﷺ) said, "A time will come when one will not care how one gains one's money, legally or illegally."

Sahih al-Bukhari Book 34, Hadith 13 Next Hadith → Abu Musa asked `Umar to admit him but he was not admitted as `Umar was busy, so Abu Musa went back. When `Umar finished his job he said, "Didn't I hear the voice of `Abdullah bin Qais? Let him come in." `Umar was told that he had left. So, he sent for him and on his arrival, he (Abu Musa) said, "We were ordered to do so (i.e. to leave if not admitted after asking permission thrice). `Umar told him, "Bring witness in proof of your statement." Abu Musa went to the Ansar's meeting places and asked them. They said, "None amongst us will give this witness except the youngest of us, Abu Sa`id Al-Khudri. Abu Musa then took Abu Sa`id Al-Khudri (to `Umar) and `Umar said, surprisingly, "Has this order of Allah's Messenger (ﷺ) been hidden from me?" (Then he added), "I used to be busy trading in markets."

Sahih al-Bukhari Book 34, Hadith 15