Musnad Ahmad 82
It was narrated that Harithah said:
Some people from Syria came to ‘Umar and said: We have acquired wealth and horses and slaves,and we want to pay zakah on them as a purification. He said: I shall do what my two predecessors did. He consulted the Companions of Muhammad (ﷺ), among whom was ‘Ali, and ‘Ali said: It is good, provided it does not become a regular tax that is taken from them after you are gone.
حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، عَنْ سُفْيَانَ، عَنْ أَبِي إِسْحَاقَ، عَنْ حَارِثَةَ، قَالَ جَاءَ نَاسٌ مِنْ أَهْلِ الشَّامِ إِلَى عُمَرَ رَضِيَ اللَّهُ عَنْهُ فَقَالُوا إِنَّا قَدْ أَصَبْنَا أَمْوَالًا وَخَيْلًا وَرَقِيقًا نُحِبُّ أَنْ يَكُونَ لَنَا فِيهَا زَكَاةٌ وَطَهُورٌ قَالَ مَا فَعَلَهُ صَاحِبَايَ قَبْلِي فَأَفْعَلَهُ وَاسْتَشَارَ أَصْحَابَ مُحَمَّدٍ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَفِيهِمْ عَلِيٌّ رَضِيَ اللَّهُ عَنْهُ فَقَالَ عَلِيٌّ هُوَ حَسَنٌ إِنْ لَمْ يَكُنْ جِزْيَةً رَاتِبَةً يُؤْخَذُونَ بِهَا مِنْ بَعْدِكَ‏.‏