Yahya related to me from Malik from Nafi from Abu Said al-Khudri that the Messenger of Allah, may Allah bless him and grant him peace, said, "Do not sell gold for gold except like for like and do not increase one part over another part. Do not sell silver for silver, except like for like and do not increase one part over another part. Do not sell some of it which is not there for some of it which is."