Musnad Ahmad 1196
It was narrated that Muhammad bin ‘Ali said:
Some people came to `Ali (رضي الله عنه) and complained about `Uthman`s tax collector. My father said to me: take this letter to `Uthman (رضي الله عنه) and tell him: The people are complaining about your tax collector, and this is the instruction of the Messenger of Allah (ﷺ) about collecting zakah, tell them to follow it. So I went to `Uthman and told him about that, He (the narrator) said: If he [`Ali] had wanted to say anything about `Uthman, he would have said it on that occasion - i.e., saying something bad.
حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَنْبَأَنَا ابْنُ عُيَيْنَةَ، عَنْ مُحَمَّدِ بْنِ سُوقَةَ، عَنْ مُنْذِرٍ الثَّوْرِيِّ، عَنْ مُحَمَّدِ بْنِ عَلِيٍّ، قَالَ جَاءَ إِلَى عَلِيٍّ رَضِيَ اللَّهُ عَنْهُ نَاسٌ مِنْ النَّاسِ فَشَكَوْا سُعَاةَ عُثْمَانَ قَالَ فَقَالَ لِي أَبِي اذْهَبْ بِهَذَا الْكِتَابِ إِلَى عُثْمَانَ فَقُلْ لَهُ إِنَّ النَّاسَ قَدْ شَكَوْا سُعَاتَكَ وَهَذَا أَمْرُ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي الصَّدَقَةِ فَمُرْهُمْ فَلْيَأْخُذُوا بِهِ قَالَ فَأَتَيْتُ عُثْمَانَ فَذَكَرْتُ ذَلِكَ لَهُ قَالَ فَلَوْ كَانَ ذَاكِرًا عُثْمَانَ بِشَيْءٍ لَذَكَرَهُ يَوْمَئِذٍ يَعْنِي بِسُوءٍ‏.‏