Sunan Abi Dawud 1647
Ibn al-Saidi said :
Umar employed me to collect the sadaqah.  When I finished doing so and gave it to him, he ordered payment to be given to me.  I said: I did only for Allah’s sake, and my reward will come from Allah.  He said: Take what you are given, for I acted (as a collector) during the time of the Messenger of Allah (May peace be upon him) and he assigned me a payment.  Thereupon, I said the same kind of thing as you have said, to which Messenger of Allah (May peace be upon him) said: When you are given something without asking for it, you should use it for your own purpose and as sadaqah.
حَدَّثَنَا أَبُو الْوَلِيدِ الطَّيَالِسِيُّ، حَدَّثَنَا اللَّيْثُ، عَنْ بُكَيْرِ بْنِ عَبْدِ اللَّهِ بْنِ الأَشَجِّ، عَنْ بُسْرِ بْنِ سَعِيدٍ، عَنِ ابْنِ السَّاعِدِيِّ، قَالَ اسْتَعْمَلَنِي عُمَرُ - رضى الله عنه - عَلَى الصَّدَقَةِ فَلَمَّا فَرَغْتُ مِنْهَا وَأَدَّيْتُهَا إِلَيْهِ أَمَرَ لِي بِعُمَالَةٍ فَقُلْتُ إِنَّمَا عَمِلْتُ لِلَّهِ وَأَجْرِي عَلَى اللَّهِ ‏.‏ قَالَ خُذْ مَا أُعْطِيتَ فَإِنِّي قَدْ عَمِلْتُ عَلَى عَهْدِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَعَمَّلَنِي فَقُلْتُ مِثْلَ قَوْلِكَ فَقَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ إِذَا أُعْطِيتَ شَيْئًا مِنْ غَيْرِ أَنْ تَسْأَلَهُ فَكُلْ وَتَصَدَّقْ ‏"‏ ‏.‏