مُوَطَّأُ مَالِكٍ
Muwatta Malik 592
Zakat
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."
Arabic/English book reference: Book 17, Hadith 12