Tawus (may Allah have mercy on him) says that Sayyiduna Umar (may Allah be pleased with him) said: What do you think, if I appoint someone as governor and command him to be just, have I fulfilled my duty? It was said: Yes. He said: No, not until I look into his work, (to see) whether he has actually acted upon my command or not.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16655