Ziyād al-Aʿjam relates: Abū Mūsā arrived bearing a letter from ʿUmar to ʿUthmān ibn Abī al-ʿĀṣ that read, “To proceed: I have reinforced you with Abū Mūsā, and you are the commander; therefore cooperate with one another. Peace.” When the siege of Istakhr dragged on, ʿUthmān ibn Abī al-ʿĀṣ sent several officers who raided the outlying districts.

Ibn Jarīr, describing the battle of Nihāwand, says: When al-Nuʿmān reached Nihāwand with his army, the enemy scattered iron caltrops in his path. He dispatched scouts who advanced unaware of the danger. One of them urged on his horse, and a caltrop lodged in its hoof so that it could not move. The man dismounted, picked up the caltrop, returned, and informed al-Nuʿmān. Al-Nuʿmān asked, “What do you advise?” They replied, “Pull back so they will think you are fleeing; they will come out after you.” Al-Nuʿmān therefore retreated; the Persians swept the caltrops aside and marched out. He then wheeled round against them, arrayed his battalions, and addressed the troops: “If I am killed, Hudhayfah will take command; if he is killed, then Jarīr al-Bajalī; and if he is killed, then Qays ibn Makshūḥ.” Al-Mughīrah felt slighted for not being named.

The Persians emerged, having chained themselves together so none would flee. The Muslims charged them. Al-Nuʿmān was struck by an arrow and killed; his brother Suwayd ibn Muqarrin wrapped him in his cloak and concealed his death until Allah granted victory, after which he handed the banner to Hudhayfah.

Allah slew Dhū al-Ḥājib—their vanguard commander—and Nihāwand was taken; thereafter the Persians never mustered another united force.

ʿUmar then sent al-Sāʾib ibn al-Aqraʿ, a freedman of Thaqīf who was an accomplished scribe and accountant, saying, “If Allah grants the people victory, distribute their spoils among them and set aside the one-fifth.” Al-Sāʾib recounts: “While I was allotting the booty, a Persian came to me and said, ‘Will you guarantee my safety concerning…’”