Sunan Ibn Majah 1787

Khalid bin Aslam, the freed slave of Umar bin Khattab, said:“I went out with Abdullah bin Umar, and a Bedouin met him and recited to him the words of Allah: 'And those who hoard up gold and silver (the money, the Zakah of which has not been paid) and spend them not in the way of Allah.' Ibn Umar Said to him: 'The one who hoards it and does not pay Zakat due on it, woe to him. But this was before the (ruling on) Zakat was revealed. When it was revealed, Allah made it a purification of wealth.' Then he turned away and said: 'I do not mind if I have the (the equivalent of) Uhud in gold, provided that I know how much it is and I pay Zakat on it, and I use it in obedience of Allah, the Mighty and Sublime'”