Nafi' (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he used to take the wealth of orphans as a loan, because he considered that safer for it than leaving it deposited, and he used to pay the zakah of their wealth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7622