Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.063.

Abdullah ibn as-Sa’di related that he came from Syria to see ‘Umar ibn al-Khattab, who asked him: “Is it true that you perform a certain job for the Muslims and you are given wages for that, but you do not accept them?” He answered: “Yes, indeed. I possess horses and slaves. I am well-off. I want my work to be a charity for the Muslims.” Then ‘Umar said: “I also wanted what you desired, but the Prophet, upon whom be peace, used to pay compensation to me. I would say to him: ‘Give it to one who is poorer than I.’ Once he gave me money and I said to him: ‘Give it to a person more needy than I.’ Then the Prophet, upon whom be peace, said: ‘Take what Allah, to Whom belongs might and majesty, gives you of His bounties without your having asked for it or being eager.’ So take it and keep it, or give it away as charity–and what is not given should not be asked for.” This is related by al-Bukhari and an-Nasa’i.