Amr bin Shu'ayb narrates from his father, from his grandfather (may Allah be pleased with him), that he (peace and blessings be upon him) said: "Whoever becomes the guardian of an orphan's wealth, let him trade with it, and not leave it idle so that it is used up by the payment of zakah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10982