From Sahih Muslim (Hadith 4054):
Abu Salid al-Khudri reported Allah's Messenger (ﷺ) as saying:Do not sell gold for gold, except like for like, and don't increase something of it upon something; and don't sell silver unless like for like, and don't increase some thing of it upon something, and do not sell for ready money something to be given later
Reference: Muslim 4054