According to Sayf and others:

The cause that ignited this encounter was that, once the Muslims had conquered Ahwaz, rescued al-‘Ala’s army from the enemy’s grasp, taken possession of the former royal seat of Istakhr together with the more recent capital—al-Mada’in—and (1) seized those cities, provinces, districts, and the many surrounding towns, they grew emboldened. At that point Yazdegerd, who had been retreating from land to land until he ended up in Isfahan, an outcast driven far from home—yet still surrounded by a retinue of his people, family, and wealth—grew furious and roused them. He wrote to the region of Nahavand and to the neighboring mountains and towns; they gathered together and exchanged letters until they mustered an army the like of which they had never assembled before.

Sa‘d therefore dispatched word to ‘Umar, informing him of the situation, while the people of Kufah simultaneously rose up against Sa‘d. They complained about him in every matter, even claiming, “He does not know how to pray properly.” The man who led this grievance was called al-Jarrah ibn Sinan al-Asadi, along with a band of supporters.

When they reached ‘Umar and laid their complaint, ‘Umar said to them, “The very proof of the ill-will you harbor (what you have within you of wickedness) (2) is that you have chosen this moment to rise against him, while he is readying himself to fight the enemies of Allah who have massed against you. Even so, I will not refrain from looking into your affair.”

He then sent Muhammad ibn Maslamah—who served as the inspector of the governors. When Muhammad ibn Maslamah arrived in Kufah, he went round the tribes, clans, and mosques of the city. Everyone spoke well of Sa‘d except the district of al-Jarrah ibn Sinan; they kept silent, neither praising nor blaming him, until he came to the clan of Banu ‘Abs. There a man named Abu Sa‘dah, Usamah ibn Qatadah, stood up and said, “Now that you have put us on oath (3), Sa‘d neither distributes the stipends equally, nor deals justly with the subjects, nor marches out with the raiding parties.”

Sa‘d invoked against him, saying, “O Allah, if he has uttered this out of falsehood, hypocrisy, and a desire for fame, then blind his sight, enlarge his family, and expose him to bewildering trials.” He went blind, ten daughters gathered around him, and whenever he heard of a woman he would persist until he reached her and groped her; if he was caught he would say, “This is the supplication of Sa‘d, the blessed man.”

Sa‘d then prayed against al-Jarrah and his companions, and each of them was struck by a bodily affliction and a loss of wealth thereafter. Meanwhile, acting on the order of ‘Umar ibn al-Khattab, Muhammad ibn Maslamah called the people of Kufah to set out against the forces of Nahavand.

Later Sa‘d, Muhammad ibn Maslamah, al-Jarrah, and his companions travelled until they reached ‘Umar. ‘Umar asked Sa‘d, “How do you pray?” He replied, “I lengthen the first two rak‘ahs and shorten the last two; I fall short in nothing of what I followed from the prayer of the Messenger of Allah, peace and blessings be upon him.”

“Such was our opinion of you, Abu Ishaq,” ‘Umar said.

Sa‘d went on to relate, “I accepted Islam as the fifth of five. In those days we had no food except the leaves of al-hublah (4) until the sides of our mouths split open. I was the first man to shoot an arrow in the path of Allah, and the Messenger of Allah, peace and blessings be upon him, once joined my mother and father in saying, ‘May my parents be ransomed for you.’”