Musnad Ahmad 937:238

Yazid narrated to us, Hammam informed us, from Qatadah, from Sa’id ibn Abi al-Hasan, from Abdullah ibn al-Samit, who said: I was with Abu Dharr when his stipend came out, and he had a slave girl with him, and she began to fulfill his needs—and he said once: "to fulfill." He said: Then there remained with him a surplus. He said: I think he said: "Seven." He said: So he ordered her to buy *fulus* (small copper coins) with it. I said: "O Abu Dharr, if only you had saved it for a need that might befall you, or for a guest who might come to you." He said: "My intimate friend (the Prophet) made a covenant with me that: 'Whatever gold or silver is tied up, it is a burning coal upon its owner on the Day of Resurrection, until he pours it out—a pouring out—in the way of Allah.'"