Salim (may Allah have mercy on him) said about his father (Sayyiduna Abdullah bin Umar (may Allah be pleased with them both)) that he had the wealth of two orphans in his care, and he began paying zakah from it. I said: Father, why do you not trade with this wealth and give it out in a profit-sharing partnership (mudarabah)? You can see that it may well be running out quickly. He said: I will keep paying zakah from it, even if only a single dirham remains. He said: Then he bought a house for the two orphans with their wealth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10988