Muhammad bin Sirin (may Allah have mercy on him) said that Umar (may Allah be pleased with him) said to Abu Musa al-Ash'ari (may Allah be pleased with him): Look into the judgements of Abu Maryam. He said: I do not suspect Abu Maryam. He said: Nor do I suspect him, but when you see wrongdoing from a litigant, then punish him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20298