Mishkat al-Masabih 1854
Ibn as-Sa‘idi said:
‘Umar employed me to collect the sadaqa , and when I finished doing so and gave it to him he ordered payment to be given me, but I said, “I did it only for God’s sake, and my reward will come from God.” He replied, 
“Take what you are given, for I acted as a collector in the time of God’s messenger, and when he assigned me a payment I said the same kind of thing as you have said, to which he replied that when I was given something without asking for it I should use it for my own purposes and as sadaqa .” 

Abu Dawud transmitted it.
وَعَن ابْن السَّاعِدِيّ الْمَالِكِي أَنه قَالَ: استعملني عمر بن الْخطاب رَضِي الله عَنْهُم عَلَى الصَّدَقَةِ فَلَمَّا فَرَغْتُ مِنْهَا وَأَدَّيْتُهَا إِلَيْهِ أَمَرَ لِي بِعُمَالَةٍ فَقُلْتُ إِنَّمَا عَمِلْتُ لِلَّهِ وَأجْرِي على الله فَقَالَ خُذْ مَا أُعْطِيتَ فَإِنِّي قَدْ عَمِلْتُ عَلَى عَهْدِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَعَمَّلَنِي فَقُلْتُ مِثْلَ قَوْلِكَ فَقَالَ لِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا أُعْطِيتَ شَيْئا من غير أَن تسْأَل فَكل وَتصدق» . رَوَاهُ مُسلم وَأَبُو دَاوُد