He formed the census of Iraq and marked out the boundaries of the territory; he arranged for the wellbeing and comfort of the public and by way of his excellent planning and carrying out his duties, he proved that God Almighty had favoured him not only with the art of war, but also with administrative expertise.
People think that after conquering a land, the Muslims did not take care of its people; on the contrary, whenever they conquered a city, they took greater care of them than all who came before them. Then there was the construction of Kufa. Owing to the fact that the climate of Midian was not suitable for the Arabs, with the permission of Hazrat Umar (ra), Hazrat Sa‘d (ra) had the Arabs dwell in a new city, wherein the Arab tribes were allocated their own quarters. A large mosque was built in the city centre, which could accommodate 40,000 worshippers at a time. Kufa was in fact a military garrison, and so it was made the dwelling place for 100,000 soldiers.
Further details of this are that after remaining in Midian for some time, Hazrat Sa‘d (ra) realised that the climate had greatly affected the Arabs. Hazrat Umar (ra) was informed of this and the instruction was given to find a suitable nearby land that they could inhabit and once the Arabs settle there, it should be made the administrative headquarters.
In accordance with this, Hazrat Sa‘d (ra) left Midian, chose a suitable land near the borders and laid the foundations for a vast city named Kufa and then settled the Arab tribes there, each to their own quarters. In the centre, a large mosque was constructed which had space for 40,000 worshippers. He also had the building for the treasury constructed near the mosque and his own palace too, which was known as Qasr-e-Sa‘d.
(Roshan Sitarey, pp. 84-88), (Sair al-Sahaba, Vol. 2, pp. 117-118) (Mujam-ul-Buldan, p. 56) (Mujam-ul-Buldan, Vol. 1, p. 610)