“they said, ‘Five gold tumors...’” This is the Philistine priests and diviners speaking and giving the solution. This seems like something they made up, there certainly is not anything in the Mosaic Law about it. The fact that the Philistines made gold mice is likely due to the fact that the plague that the Philistines were experiencing had something to do with mice (or rats). In fact, some of the early Greek translations include that the plague was carried by mice, although there is no such information in the Hebrew texts.