It is narrated that Sayyiduna Abdullah al-Muzani (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) forbade breaking the coinage of the Muslims that is current among them, unless there is some defect in it, (that is,) that a dirham be broken and made into raw silver, or a dinar be broken and made into dirhams.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14066