مسند امام احمد بن حنبل · Hadith 280

It was narrated that 'Abdullah bin as-Sa'di said: 'Umar (RA) said to me: Haven't been told that you do certain tasks for the state, then when you are given payment you do not accept it? He said: Ves. He said: Why do you do that? He said: i am well off and I have slaves and horses; I want my work to be a charity to the Muslims. He said: Do not do that, for I used to do what you are doing, and the Messenger of Allah (PBUH) would give me payment, and I would say: Give it someone who is more in need of it than me. And he said: Whatever of this wealth comes to you when you are not hoping for it or asking for it, take it, otherwise do not hanker after it."