Esther 3:4 (ESV)

4 And when they spoke to him day after day and he would not listen to them, they told Haman, in order to see whether Mordecai’s words would stand, for he had told them that he was a Jew.

Relationships among royal officials were often characterized by envy and rivalry. Moreover, Mordecai’s stubborn refusal would have been hard for his fellow officials to bear with, since they were obligated to bow before Haman day after day! Consequently, they brought Mordecai’s disobedience to Haman’s attention.