Hadith 505920
Chapter 17: Zakat - كتاب الزكاة
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."
Reference Muwatta Malik 505920
USC-MSA web (English) reference : Book 17, Hadith 12