It is mentioned in the Sahih of Dawūd bin Zarbi that one of the followers of Imam Sajjad (a.s.) had requested Imam Sajjad (a.s.) to recommend him to the ruler of Madinah, Dawūd bin ‘Ali for an appointment in the governmental post. Imam Sajjad (a.s.) replied: “I will never do such a thing.”
On hearing this he thought that Imam (a.s.) was unwilling to recommend him because he might be worried that some injustice would be caused by him. So he approached Imam (a.s.) and began to promise and take oaths that he would never do any kind of unjust act and said that he would not do anything except help the people. Imam (a.s.) looked at the sky and began to weep. Then he said that going to the sky was easier. It was apparent that Imam (a.s.) meant to say that once a person accepts employment of an oppressor it is impossible for him not to oppress.