Musnad Ahmad 937:95

'Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, from Sa'id ibn Abi al-Hasan, from 'Abdullah ibn al-Samit: That he was with Abu Dharr, and his stipend came out, and he had a slave girl with him, and she began to fulfill his needs. He said: Seven [dirhams] remained with her. He said: So he ordered her to buy *fulus* (small copper coins) with it. I said to him: If only you had saved it for a need that might befall you, or for a guest who might descend upon you. He said: My intimate friend (the Prophet) made a covenant with me: "Whatever gold or silver is tied up, it is a burning coal upon its owner until he spends it in the way of Allah, the Almighty and Majestic."