Allah Ta‘ala had blessed Hazrat Sa’d (radhiyallahu ‘anhu) by allowing the conquer of ‘Iraq to take place at his hands. It was the armies under his command that defeated the Romans in Qaadisiyyah. However, Hazrat Sa’d (radhiyallahu ‘anhu) could not personally lead the army in this specific battle, as he was ill and sores had broken out across his entire body, and so he informed the people of his illness and excused himself from joining them in fighting against the kuffaar.