Yarfa' (the servant) said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said to me: "I have placed myself, with respect to the treasury, in the position of the guardian of an orphan: if I am in need, I take from it; when I am well off, I return it; and if I have no need of it, I refrain from it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13011