After the Battle of al-Qadisiyyah and the Siege of Ctesiphon (637), Sa'd served as the supreme commander of the Rashidun army in Iraq, which conquered Khuzestan and built the garrison city of Kufa.

[2] During the First Fitna, Sa'd was known for leading the neutral faction that contained the majority of the companions of Muhammad and their followers, who refused to be involved in the civil war.

[8] Sa'd's mother opposed her son's conversion and threatened to go on a hunger strike until he left Islam, but he did not heed her threat and she finally yielded due to his insistence.

His first operation occurred nine months after the migration, when he was tasked with leading 20 men to raid a Qurayshi caravan that passed Kharrar, located between Al-Juhfa and Mecca.

[14] During a minor reconnaissance operation under Ubayda ibn al-Harith in Rabigh shortly before the Battle of Badr, the team caught the attention of opposing Qurayshi fighters that began to chase them.

He killed a Qurayshi champion named Sa'id ibn al-As and retrieved a sword known as Dha al-Kutayfah (ذا الكُتَيفَة), which he presented to Muhammad as a prize of war.

[9] The group was surrounded by enemy cavalry under Khalid as the Muslim fighters formed a close defensive formation and Sa'd shot his arrows next to Muhammad, who suffered an injury to his shoulder.

[9] The outnumbered and encircled Muslims fought until most of them were killed, except Muhammad, Talhah, Abu Dujana, and Sa'd, who tried to assist his comrades with his bow, despite the close combat.

Abu Bakr dispatched his elite forces under Usama ibn Zayd to pacify the northern border, while he gathered the rest of the army, including Sa'd, to engage the rebel invaders led by Tulayha in the Battle of Zhu Qissa.

[24] This development caused Umar to change his plans, instructing Sa'd to march to Iraq with 6,000 soldiers, while also instructing the Rashidun armies in Iraq to merge with Sa'd's forces, the forces of Arfajah, who brought 400 to 700 Azd cavalry,[26] Jarir ibn Abdullah of al-Bajali and al-Muthanna ibn Haritha of the Banu Shayban, as those three commanders have just defeated the Sassanid vanguard in the Battle of Buwaib.

[9][27] According to Tabari's account, the Persian faction of the Sassanid civil war that steered the policies of the young Yazdegerd III was at odds with Rostam, the commander of the empire's most powerful army.

Rostam urged patience and protracted warfare instead of outright assault on the Arab troops and exchanged letters with Zuhra ibn Hawiyah with the intention of making peace.

Tabari's narrative states that Rostam was prepared to convert in order to avoid military confrontation, but other factions in the Sassanid government refused to agree to such terms, and battle became an inevitability.

[28] Islamic sources state that Sa'd sent a series of hostile emissaries to taunt Rostam while waiting to receive reinforcements sent by Abu Ubayda ibn al-Jarrah, who had just won the Battle of Yarmuk.

[9][30] As Rostam's army marched to the battlefield, Sa'd sent a dozen horsemen as scouts, led by Tulayha and Amr ibn Ma'adi Yakrib, who disguised themselves as Iraqi locals.

Modern scholars hold that most details in works like al-Tabari's History of the Prophets and Kings consist of embellishments, with narrators recounting legendary tales of their fellow tribesmen, such as Sayf ibn Umar's emphasis on the heroics of al-Qa'qa, both of them members of the Banu Tamim.

[41][note 7] After Rostam's death, al-Qa'qa and his Tamim cavalry were surrounded behind enemy lines, while the Muslim army carried out Sa'd's order to advance.

[38] As the Sassanid casualties mounted, they were finally routed and fled towards the river of Ateeq, where they were subject to further slaughter by the Tamim cavalry led by Zahra ibn Hawiyah.

[29] Al-Antioch had gathered some Byzantine men as his allies, along with a man named Syaharijah and Arab Christian warriors from the tribes of Iyad, Taghlib, and an-Nimr.

[46] Sa'd appointed Abdullah ibn Mu'tam as the commander of the forces set to attack Mosul, with Rib'i bin al-Afkal al-Inazi as the vanguard.

[29] As Yazdegerd raised further resistance forces, Sa'd's troops under Arfajah chased them, sending the vanguard led by a Tamim warrior named Hurqus ibn Zuhayr as-Sa'di (known as Dhu al-Khuwaishirah at-Tamimi, the first Kharijite in history.

[53] After Sa'd settled into Kufa, he instructed Hashim ibn Utbah to bring his forces towards locations in Khuzestan centered around Ahvaz to face Hormuzan, a fugitive commander who survived the Battle of al-Qadisiyyah.

[55][better source needed] He refused all of Sa'd's offers of hospitality, and handed him a missive from Umar reminding him that the citadel should be available to the public, suggesting that he move his house.

[50] Later, on the eve of the Battle of Nahavand, Umar gathered a war council consisting of Sa'd, Uthman, Ali, Talha, Zubayr, and Abbas ibn Abd al-Muttalib.

This law, which is not described in the Qu'ran, is largely based on a tradition in which a gravely ill Sa'd requests Muhammad's guidance in determining how much of his wealth he should bequeath to charity.

[75] Maurice Gajan speculates that the local traditions about Sa'd are linked to some Muslim traders from the West Asia establishing small communities in the coastal towns of Quanzhou, Guangzhou, and Yangzhou during early medieval periods.

[82][27][9][45] Sa'd's leadership was characterized by his liberal stance towards lower officers, as Sa'd usually give his subordinates freedom to express their ideas,[45][27] whether by relying on the wisdom of strategy experts such as al-Muthanna, Asim, al-Mughira and Arfajah[26] or by allowing decisive commanders like Tulayha, al-Qa'qa, and Amr ibn Ma'adi Yakrib to mount their own initiatives during battle.

[9] Sa'd intensified his correspondences with Umar by creating an communication system between Medina and the frontline, giving the caliph a comprehensive understanding of the developments in Qadisiyyah.

Sa'd also married Makita bint Amr of the Banu Bahra' clan of the Quda'a tribe, which claimed descent from Ma'ad ibn Adnan.

[81] Asad Q. Ahmed states that Sa'd's children and grandchildren gained prominence during the early years of the Umayyad caliphate and were particularly popular with the southern Arab tribes due to his marriage alliances.