Musnad Ahmad 371
It was narrated that Ibn as Sa`idi al-Maliki said:
`Umar bin al Khattab (رضي الله عنه) appointed me in charge of the zakah, and when had finished with it and handed it over to him, he ordered that I be given some remuneration. I said: I only did it for the sake of Allah and my reward is with Allah. He said: Take what is given to you. I was appointed to do some work at the time of the Messenger of Allah (ﷺ) and he gave me some remuneration, and I said the same as you have said, but the Messenger of Allah (ﷺ)  said to me: `If you are given something without asking for it, then take it and give some in charity.”
حَدَّثَنَا حَجَّاجٌ، حَدَّثَنَا لَيْثٌ، حَدَّثَنِي بُكَيْرُ بْنُ عَبْدِ اللَّهِ، عَنْ بُسْرِ بْنِ سَعِيدٍ، عَنْ ابْنِ السَّاعِدِيِّ الْمَالِكِيِّ، أَنَّهُ قَالَ اسْتَعْمَلَنِي عُمَرُ بْنُ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ عَلَى الصَّدَقَةِ فَلَمَّا فَرَغْتُ مِنْهَا وَأَدَّيْتُهَا إِلَيْهِ أَمَرَ لِي بِعِمَالَةٍ فَقُلْتُ لَهُ إِنَّمَا عَمِلْتُ لِلَّهِ وَأَجْرِي عَلَى اللَّهِ قَالَ 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 شَيْئًا مِنْ غَيْرِ أَنْ تَسْأَلَ فَكُلْ وَتَصَدَّقْ‏.‏