Musannaf Ibn Abi Shayba 13:233

Ghundar narrated to us, from Shu'bah, from 'Amarah ibn Abi Hafsah, from al-Mughirah ibn Hunayn, who said: I asked Ali about vessels of gold mixed with silver, [asking] if they may be sold for silver. He said—and he gestured with his head like this—meaning: There is no harm in it.