Rabi Ibn Amer رضي الله ﺗﻌﺎﻟﯽٰ عنه explains the mission of Islam to the Persian King, Rustum:
At the head of the Persian army Rustam marched against Qadisiyya and encamped on the east bank of the Ateeq. The Muslim forces lay entrenched at Qadisiyya on the west bank of the Ateeq.
Rustam the Commander-in-Chief of the Persian forces sent a message to the Muslim Commander Saad bin Abi Waqqaas رضي الله ﺗﻌﺎﻟﯽٰ عنه asking him to send an emissary for talks. Saad deputed Rabi bin Amir as the envoy.
Rabi crossed the bridge and made for the camp of Rustam. Rabi appeared before Rustam wearing a coat of shining mail over which was wrapped a coarse woollen cloak. Around his head was a veil held by thongs of a camel's girth. His sword hung at his side in a sheath of coarse cloth. In his right hand he carried his spear. Rabi mounted on a shaggy horse arrived at the edge of the carpet at which Rustam and his couriers were seated.
The Persians wanted Rabi to lay aside his arms. Rabi said, "I have not come to you to lay down my weapons. You invited me, and I have come, if you do not wish me to come the way I like, I shall return."
Rustam asked his men to let the Muslim come the way he wished.
When Rustum, the king of persia asked the Muslims, why had they come to Persia?
Rabiah Ibn 'Amir said “Allah Azza wa jal has sent us to deliver you from worshiping the creation to worshiping the Creator of the creation and to deliver you from the constriction of this world to the vastness of this world and the hereafter and from the oppression of the religions to the justice of Islam. Allah Azza wa jal has sent us to save you from worshiping each other.”
Rabi said that their mission was to spread Islam. He said, "If you accept Islam we are brothers and there is peace between us; if you refuse we fight you and leave things to God."
"What do you expect in return", asked Rustam.
Rabi said, "Victory if we survive, and Paradise if we die fighting in the way of Allah".
Rustam said that he should be allowed some time to think over the matter further.
Rabi said that according to a tradition of the Holy Prophet he could give him a time of three days.
"Are you their chief", asked Rustam.
Rabi said, "No, but the Muslims are like one body, and the lowest is equal to the highest."
Hadrat Abu Bakr, Umar, Usman, Ali (ra) 4 Vol. Set by Professor Masud-ul-Hasan, Islamic Publications, Lahore, Pakistan