Yusuf bin Mahak (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "In the wealth of the orphan," or he said: "In the wealth of the orphans, trade, so that the wealth is not used up by charity, that is, by paying zakah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10983