Musannaf Ibn Abi Shayba 28:386

Waki’ narrated to us, he said: Sufyan narrated to us, from Abu Ishaq, from Harithah ibn Mudarrib al-‘Abdi, who said: Umar said: "I have placed myself in relation to the wealth of Allah in the position of the guardian of an orphan's wealth. If I am wealthy, I abstain from it; and if I am in need, I consume from it in a reasonable manner."