Sahih Muslim Book 10, Hadith Number 3845.
Chapter : Sale of wheat like for like.
Abu Salid al-Khudri reported Allah’s Messenger (may peace be upon him) 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 something of it upon something, and do not sell for ready money something to be given later.