Maliks Muwatta Book 17, Hadith Number 14.

Section : The Zakat on the Property of Orphans and Trading on their Behalf.

Yahya related to me from Malik that he had heard that A’isha, the wife of the Prophet, may Allah bless him and grant him peace, used to give the property of the orphans that were in her house to whoever would use it to trade with on their behalf.