After the battle of Nihawand, many Persians, men, women, and children were taken as captives by the Muslims. The captives were sold as slaves. One of these siaves was Firoz alias Abu Lulu. He was purchased by Mughirah Shu'bah the Governor of Basra. This Firoz was a craftsman, a carpenter, an iron smith and a painter. Umar did not allow non-Muslim adult captives to reside in Madina. Mughirah sought special permission for the residence of Firoz in Madina on the ground that as he was a skilled craftsman, he would be of service to the people. Umar gave the permission as a special case.

One day, Firoz waited on Umar and complained that the tax which his master Mughirah was exacting from him was too high. He wanted the Caliph to reduce the levy. Umar enquired what work did he do. He said that he worked as a carpenter, painter, and an ironsmith. He added that he could make windmills as well. Umar next enquired as to the amount of the tax that he was required to pay to his master. He said that he had to pay two dirhams a day. Umar said that keeping in view the lucrative nature of the jobs done by him, the levy of two dirhams a day was prima facie not excessive. Umar said that he would, however, write to Mughirah, and examine the question further in the light of what Mughirah said. That did not satisfy Firoz, and he went away sulking.

Umar wrote to Mughirah, and in reply Mughirah quoted facts and figures to establish that what he took from his slave was by no means excessive. When Firoz called on Umar again, Umar explained to him that as the levy was not excessive, no reduction therein was called for that made Foroz angry. In order to humour Firoz, Umar said, "I understand you make windmills; make one for me as well." In a sullen mood, Firoz said, "Verily I will make such a mill for you, that the world would talk about it." As Firoz went away, the Caliph told the people around him that the Persian slave had threatened him.

There were Persian children slaves in Madina. Seeing them, Firoz would say, "You have been enslaved at such a tender age. This Umar sees eaten my heart. I will take his heart out". He made for himself a dagger with a very sharp edge and smeared it with poison.

On the 1st of November 644 A.D. at the time of the morning prayer, Firoz went with his dagger to the Prophet's mosque and hid himself in a corner in one of the recesses of the mosque. When the faithful stood for prayer after straightening the lines, and Umar took up his position as the Imam to lead the prayer, Firoz emerged from his place of hiding and rushed at Umar. Firoz struck Umar six consecutive blows with his dagger, and Umar fell on the floor profusely bleeding.

Other persons rushed at Firoz, but he had the fury and frenzy of a desperate man about him. He struck right and left, and thirteen Muslims were wounded, some of them fatally, before Firoz could be overpowered. At last realising that he could not escape, Firoz stabbed himself to death with his own dagger.