Musnad Ahmad 937:172

Yazid narrated to us, Hammam ibn Yahya informed us, from Qatadah, from Sa'id ibn Abi al-Hasan, from Abdullah ibn al-Samit, who heard Abu Dharr say: "My intimate friend, may Allah bless him and grant him peace, made a covenant with me: 'Whatever gold or silver is tied up (hoarded), it is a branding iron upon its owner until he pours it out in the way of Allah, a pouring out.'"