Musnad Ahmad 1302:19

Yahya ibn Sa'id narrated to us, from Malik, from Zayd ibn Aslam, from 'Ata' ibn Yasar, that Mu'awiyah bought a silver drinking vessel for less than its weight or more. He said: Abu al-Darda' said, "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the likes of this, unless it is equal for equal."