To confront the numerous and well-equipped Persians was a most daunting task. The most powerful force had to be mustered. Umar sent despatches to Muslim governors throughout the state to mobilize all able-bodied persons who had weapons or mounts, or who had talents of oratory and other skills to place at the service of the battle.
Bands of Mujahidin then converged on Madinah from every part of the Muslim domain. When they had all gathered, Umar consulted the leading Muslims about the appointment of a commander-in-chief over the mighty army. Umar himself thought of leading the army but Ali suggested that the Muslims were in great need of him and he should not endanger his life. Sad was then chosen as commander and Abdur-Rahman ibn Awl, one of the veterans among the Sahabah said:
"You have chosen well! Who is there like Sad?" Umar stood before the great army and bade farewell to them. To the commander-in-chief he said:
"O Sad! Let not any statement that you are the uncle of the Messenger of God or that you are the companion of the Messenger of God distract you from God. God Almighty does not obliterate evil with evil but he wipes out evil with good.
"O Sad! There is no connection between God and anyone except obedience to Him. In the sight of God all people whether nobleman or commoner are the same. Allah is their Lord and they are His servants seeking elevation through taqwa and seeking to obtain what is with God through obedience. Consider how the Messenger of God used to act with the Muslims and act accordingly..."
Umar thus made it clear that the army was not to seek conquest for the sake of it and that the expedition was not for seeking personal glory and fame.
The three thousand strong army set off. Among them were ninety nine veterans of Badr, more than three hundred of those who took the Pledge of Riffwan (Satisfaction) at Hudaybiyyah and three hundred of those who had participated in the liberation of Makkah with the noble Prophet. There were seven hundred sons of the companions. Thousands of women also went on to battle as auxiliaries and nurses and to urge the men on to battle.
The army camped at Qadisiyyah near Hira. Against them the Persians had mobilized a force of 12O,OOO men under the leadership of their most brilliant commander, Rustum.
Umar had instructed Sad to send him regular despatches about the condition and movements of the Muslim forces, and of the deployment of the enemy's forces. Sad wrote to Umar about the unprecedented force that the Persians were mobilizing and Umar wrote to him:
"Do not be troubled by what you hear about them nor about the (forces, equipment and methods) they would deploy against you. Seek help with God and put your trust in Him and send men of insight, knowledge and toughness to him (the Chosroes) to invite him to God... And write to me daily."
Sad understood well the gravity of the impending battle and kept in close contact with the military high command in Madinah. Although commander-in-chief, he understood the importance of shura.
Sad did as Umar instructed and sent delegations of Muslims first to Yazdagird and then to Rustum, inviting them to accept Islam or to pay the jizyah to guarantee their protection and peaceful existence or to choose war if they so desired.
The first Muslim delegation which included Numan ibn Muqarrin was ridiculed by the Persian Emperor, Yazdagird. Sad sent a delegation to Rustum, the commander of the Persian forces. This was led by Rubiy ibn Aamir who, with spear in hand, went directly to Rustam's encampment. Rustam said to him:
"Rubiy! What do you want from us? If you want wealth we would give you. We would provide you with provisions until you are sated. We would clothe you. We would make you become rich and happy. Look, Rubiy! What do you see in this assembly of mine? No doubt you see signs of richness and luxury, these lush carpets, fine curtains, gold embroidered wails, carpets of silk...Do you have any desire that we should bestow some of these riches which we have on you?"
Rustum thus wanted to impress the Muslim and allure him from his purpose by this show of opulence and grandeur. Rubiy looked and listened unmoved and then said:
"Listen, O commander! Certainly God has chosen us that through us those of His creation whom He so desires could be drawn away from the worship of idols to Tawhid (the affirmation of the unity of God), from the narrow confines of preoccupation with this world to its boundless expanse and from the tyranny of rulers to justice of Islam.
"Whoever accepts that from us we are prepared to welcome him. And whoever fights us, we would fight him until the promise of God comes to pass." "And what is the promise of God to you?" asked Rustum. "Paradise for our martyrs and victory for those who live."
Rustum of course was not inclined to listen to such talk from a seemingly wretched person the likes of whom the Persians regarded as barbaric and uncivilized and whom they had conquered and subjugated for centuries.
The Muslim delegation returned to their commanderin-chief. It was clear that war was now inevitable. Sad's eyes filled with tears. He wished that the battle could be delayed a little or indeed that it might have been somewhat earlier. For on this particular day he was seriously ill and could hardly move. He was suffering from sciatica and he could not even sit upright for the pain.
Sad knew that this was going to be a bitter, harsh and bloody battle. And for a brief moment he thought, if only... but no! The Messenger of God had taught the Muslims that none of them should say, "If....." To say "If....." implied a lack of will and determination and wishing that a situation might have been different was not the characteristic of a firm believer. So, despite his illness, Sad got up and stood before his army and addressed them. He began his speech with a verse from the glorious Quran:
"And indeed after having exhorted (man), We have laid it down in all the books of Divine wisdom that My righteous servants shall inherit the earth." Surah al-Anbiyaa, 21:1O5).
The address over, Sad performed Salat az-Zuhr with the army. Facing them once again, he shouted the Muslim battle cry "Allahu Akbar" four times and directed the fighters to attack with the words:
"Hayya ala barakatillah Charge, with the blessings of God." Standing in front of his tent, Sad directed his soldiers and spurred them on with shouts of Allahu Akbar (God is Most Great) and La hawla wa la quwwata ilia billah (there is no power or might save with God). For four days the battle raged. The Muslims displayed valor and skill. But a Persian elephant corps wrought havoc in the ranks of the Muslims. The ferocious battle was only resolved when several renowned Muslim warriors made a rush in the direction of the Persian commander. A storm arose and the canopy of Rustam was blown into the river. As he tried to flee he was detected and slain. Complete confusion reigned among the Persians and they fled in disarray.
Just how ferocious the battle was can be imagined when it is known that some thirty thousand persons on both sides fell in the course of four days' fighting. In one day alone, some two thousand Muslims and about ten thousand Persians lost their lives.
The Battle of Qadisiyyah is one of the major decisive battles of world history. It sealed the fate of the Sasanian Empire just as the Battle of Yarmuk had sealed the fate of the Byzantine Empire in the east.
Two years after Qadisiyyah, Sad went on to take the Sasanian capital. By then he had recovered his health. The taking of Ctesiphon was accomplished after a brilliant crossing of the Tigris river while it was in flood. Sad has thus gone down in the annals of history as the Hero of Qadisiyyah and the Conqueror of Ctesiphon.
He lived until he was almost eighty years old. He was blessed with much influence and wealth but as the time of death approached in the year 54 AH, he asked his son to open a box in which he had kept a course woolen jubbah and said: "Shroud me in this, for in this (jubbah) I met the Mushrikin on the day of Badr and in it I desire to meet God Almighty."