Amr bin Shu'ayb (may Allah have mercy on him) narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "Whoever becomes the guardian of an orphan who has wealth should trade with it on his behalf, and should not leave it so that zakah consumes it."
Al-Muthanna (bin al-Sabbah) and Mandal (bin Ali) are both not strong (narrators).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7339