Sa'id bin al-Musayyib (may Allah have mercy on him) said that Umar bin al-Khattab (may Allah be pleased with him) said: Trade with the orphan's wealth, lest zakah consume it.
This chain is sahih (authentic), and it has supporting reports from Umar (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7340