Mu'awiyah was worried about Malik's going to Egypt, for he knew that Malik would save it. So, Mu'awiyah thought about a way to kill him.
Mu'awiyah used to mix honey with poison to kill his enemies. Mu'awiyah imported poisons from Rome. The Romans allowed Mu'awiyah to import them because they knew that he would use them to kill the Muslims.
Amru bin al-Aas said:
I know a man. The man lives in al-Qilzim City on the borders of Egypt. He has vast lands. Certainly Malik al-Ashtar will pass through the city and stop in it to rest.
Let's send a man to tell him to kill al-Ashtar and we won't tax him for life.
Thus Mu'awiyah's delegate quickly set out for Egypt taking the poisoned honey to persuade the man to kill Malik al-Ashtar.