Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) entered the treasury (Bayt al-Mal) and called for the dross (waste metal) that was in it, then lit a fire over it until, when it had melted, he said: "Where is the one who asked about al-muhl? This is al-muhl."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9051