Muwatta Malik:Zakat:Hadith 601
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ اتَّجِرُوا فِي أَمْوَالِ الْيَتَامَى لاَ تَأْكُلُهَا الزَّكَاةُ ‏.‏

Yahya related to me from Malik that he had heard that Umar ibn al-Khattab said, “Trade with the property of orphans and then it will not be eaten away by zakat.”