Is it authentically established that Mu'awiyah ibn Abi Sufyan used to hoard gold and silver, and how do we refute that?

The forbidden treasure is wealth on which zakat has not been paid. As for wealth on which zakat has been paid, it is not a treasure. Saving for a need is permissible in Islamic law, and the Prophet, peace be upon him, saved for his family their sustenance for a year. The hadith also indicates the permissibility of saving provisions for one's family, and that preparing for a need is commendable both religiously and rationally. The Prophet's command to Ka'b ibn Malik, may Allah be pleased with him, to keep some of his wealth upon his repentance also demonstrates the permissibility of saving.

Despite his abundant spending, Mu'awiyah, may Allah be pleased with him, governed the people with excellent policies and was among the best rulers whose justice outweighed their injustice. The subjects unanimously pledged allegiance to him, and the Muslims lived with him in comfort and justice. The majority of the Companions and scholars disagreed with Abu Dharr, may Allah be pleased with him, regarding the definition of treasure.