Al-Qadisiyyah

In the caliphate of 'Umar, the Muslim armies turned to the Persian empire. 'Umar chose Sa'd to command the main army against the Persians in Iraq, writing to him to fear Allah, to be patient, and to keep his troops from sin.

At al-Qadisiyyah the Muslims faced the Persian commander Rustam with a large army and war elephants. The battle lasted several days. Sa'd was too ill with painful boils to ride, and directed the army from a building overlooking the field.

The Muslims won, Rustam was killed, and the road to the Persian capital Ctesiphon (al-Mada'in) lay open. Sa'd later led the army into the city and founded the garrison city of Kufa. 'Umar remained attentive to him, and later dismissed him from Kufa over complaints, which 'Umar investigated, without finding wrongdoing.