Imaam Malik rahimullah narrates in his Muwatta in Book 37, Hadith 1464
Abu’d-Darda wrote to Salman al-Farsi, “Come immediately to the holy land.”
Salman wrote back to him, “Land does not make anyone holy. Man’s deeds make him holy. I have heard that you were put up as a doctor to treat and cure people. If you are innocent, then may you have delight! If you are a quack, then beware lest you kill a man and enter the Fire!”
When Abu’d-Darda judged between two men, and they turned from him to go, he would look at them and say, “Come back to me, and tell me your story again. A quack! By Allah!”
حَدَّثَنِي مَالِكٌ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّ أَبَا الدَّرْدَاءِ، كَتَبَ إِلَى سَلْمَانَ الْفَارِسِيِّ أَنْ هَلُمَّ إِلَى الأَرْضِ الْمُقَدَّسَةِ فَكَتَبَ إِلَيْهِ سَلْمَانُ إِنَّ الأَرْضَ لاَ تُقَدِّسُ أَحَدًا وَإِنَّمَا يُقَدِّسُ الإِنْسَانَ عَمَلُهُ وَقَدْ بَلَغَنِي أَنَّكَ جُعِلْتَ طَبِيبًا تُدَاوِي فَإِنْ كُنْتَ تُبْرِئُ فَنِعِمَّا لَكَ وَإِنْ كُنْتَ مُتَطَبِّبًا فَاحْذَرْ أَنْ تَقْتُلَإِنْسَانًا فَتَدْخُلَ النَّارَ . فَكَانَ أَبُو الدَّرْدَاءِ إِذَا قَضَى بَيْنَ اثْنَيْنِ ثُمَّ أَدْبَرَا عَنْهُ نَظَرَ إِلَيْهِمَا وَقَالَ ارْجِعَا إِلَىَّ أَعِيدَا عَلَىَّ قِصَّتَكُمَا مُتَطَبِّبٌ وَاللَّهِ .
Here the companion Salman al-Farsi (may Allah be pleased with him) would advise against quackery and acting like a quack and this is despite Salman not being a doctor and not being certain of the accusation but clarifying it directly with the one being accused of it and warning him against it. The companion Abu Darda (may Allah be pleased with him) accepting the advice and being double careful of not falling into something similar to quackery.
In the likes of this advice is one’s rectification. Allah, the Most High, knows best.