God selected Musa, alayhis-salam, for Prophethood and commanded him to go with his brother, Harun, to the tyrant Fir'awn (Pharaoh) and frighten him with the threat of Divine chastisement.
God commanded Musa to speak gently to Fir'awn at first and invite him to worship of the One God.
Musa and his brother, Harun, set off for Fir'awn palace and, after waiting a while, they finally went in to see him. Just as God had commanded them, they began their conversation with him gently and softly. They told him:
"O Fir'awn, we have brought a message from the Lord and Exalter of the world and all its inhabitants, and we intend to deliver the people from injustice and oppression and lead them towards knowledge and light.
"We want you to stop your injustice and arrogance and stop tyrannizing the people. We are Prophets of the Supreme Lord and we have been commanded to make the people aware of their God and to call them to obey Him."
When Fir'awn heard what Musa said, he became very agitated and, in order to silence Musa, he demanded of him:
"Who is this Lord who has given you this message?"
Musa quietly but firmly replied:
"My Lord is the One Almighty God Who has created the world and its creatures in all its order and magnificence, Who has guided everything towards its special path of perfection."
Fir'awn, who understood very well what Musa had said, pretended to be stupid and said:
"But is there any God besides me? Is not your life and death in my power? Don't you see that the whole of Egypt obeys my command? And that all these streams, gardens and fields belong to me?
"O Musa, if you follow a lord other than me, if you obey anyone other than me, I'll have you thrown in prison."
Musa was a Prophet of God and was calm and collected, without the slightest trace of fear in his voice, as he said:
"My Lord and Exalter is the Lord and Exalter of the whole universe, the Creator of the heavens and the earth. He has made us His Prophets for us to deliver His message to the people and deliver them from tyranny and oppression."
Fir'awn saw that he was being belittled and humiliated by Musa's calmness and the strength of Musa's faith. He said to Musa:
"If you are telling the truth, give me a sign of this Lord of yours.
Fir'awn looked on.
Musa, the Holy Prophet of God, put his hand inside his jacket and immediately brought it out. His hand was now shining like a star. Then he threw his walking staff at the feet of Fir'awn. At the order of God, the staff became a huge, angry snake. Fir'awn took one look at the snake and fled from his throne, screaming.
Musa bent down, took hold of the snake, and once again it became a staff.
Fir'awn, who had turned completely white and was shaking violently, said to those around him:
"Did you see what a magician this man is? These two men with their magic want to turn you out of your homes."
source : http://www.maaref-foundation.com