Musnad Ahmad 2:289

Hajjaj narrated to us, Layth narrated to us, Bukayr ibn Abdullah narrated to me, from Busr ibn Sa’id, from Ibn al-Sa’idi al-Maliki, that he said: Umar ibn al-Khattab employed me to collect the charity (zakat). When I finished it and delivered it to him, he ordered a payment for my labor for me. I said to him: "I only worked for the sake of Allah, and my reward is with Allah." He said: "Take what you are given, for I worked during the time of the Messenger of Allah, peace and blessings of Allah be upon him, and he employed me. I said the same as you said, and the Messenger of Allah, peace and blessings of Allah be upon him, said to me: 'If you are given something without asking for it, then eat and give in charity.'"