ʿAlī was 22 or 23 years old when he migrated to Medina. Shortly after his arrival, the Prophet told ʿAlī that he (the Prophet) had been ordered by God to give his daughter Fāṭimah to ʿAlī in marriage. This union affected the entire history of Islam, for from it were born a daughter, Zaynab—who played a major role during the Umayyad period in claiming the rights of the family of the Prophet after her brother Ḥusayn was killed in Iraq—and two sons, Ḥasan and Ḥusayn. The latter two are the ancestors of those known as sharīf or sayyid (meaning “noble” and “master” respectively)—that is, descendants of the Prophet and thus, in the eyes of some Muslims, legitimate heirs to leadership of the Islamic community. Ḥasan and Ḥusayn also became the second and third imams of the Shiʿah (respectively) after ʿAlī. Although polygyny was permitted, ʿAlī did not marry another woman while Fāṭimah was alive, and his marriage to her possesses a special spiritual significance for all Muslims because it is seen as the marriage between the greatest saintly figures surrounding the Prophet. The Prophet, who visited his daughter nearly every day, became even closer to ʿAlī, once telling him, “You are my brother in this world and the Hereafter.” After Fāṭimah’s death, ʿAlī married other wives and fathered many other children.

During this period ʿAlī was given several important assignments, such as reciting to a large gathering of pilgrims in Mecca in 630 a portion of the Qurʾān that declared that Muhammad and the Islamic community were no longer bound by agreements made earlier with polytheists. One year later ʿAlī was sent to Yemen to spread the teachings of Islam. The Prophet also designated him as one of the scribes who would write down the text of the Qurʾān, which had been revealed to Muhammad during the previous two decades. ʿAlī’s role in the establishment of the written version of the Qurʾān is among the most important of his contributions to Islam.

ʿAlī was also deeply involved in the military defense of the Islamic community, according to both Sunni and Shiʿi sources. The Quraysh sought to destroy the community in Medina in a series of attacks that are known in Islamic history as ghazwah (“raid” or “conquest”). ʿAlī participated in all but one of these battles, and he was commander at the battles of Fadak in 628 and Al-Yamān in 632. He also had the special role of protecting Muhammad at the battles of Uḥud in 625 and Ḥunayn in 630. He fought the leading warrior of the Quraysh, Talḥah ibn Abī Talḥah, who boasted that he would defeat any Muslim sent against him. When Talḥah himself was defeated, he pleaded for mercy from ʿAlī, saying “Karrama Allāhu wajhahu” (“May God illuminate his face with nobility”). This benediction became one of ʿAlī’s titles; used especially by Sunnis, it is usually accompanied by other customary formulae of peace and benediction.

The traditional accounts of ʿAlī’s strength and courage in these battles and his yearning for justice made him an epitome of chivalry throughout the Islamic world. In the Battle of Khaybar in 629, against a group of Medinese Jews who, having reached agreement with the Muslims and then broken their word, had barricaded themselves in a fort, ʿAlī is said, according to a very popular legend, to have torn off the door of the fort with one hand and used it as a shield. According to another legend, the archangel Gabriel, speaking to the Prophet and referring to Dhū al-fiqār, a sword that ʿAlī received from Muhammad, stated: “There is no chivalrous person but ʿAlī. There is no sword but Dhū al-fiqār.”

As Islam began to spread throughout Arabia, ʿAlī helped to establish the new Islamic order. He was instructed to write down the Hudaybiyyah agreement, the peace treaty between the Prophet and the Quraysh in 628. When Muhammad finally conquered Mecca in 630, he asked ʿAlī to guarantee that the conquest would be bloodless; this was accomplished as a result of the surrender of the Meccans and Muhammad’s forbidding the victorious Muslims from taking revenge on the Meccans, a command that ʿAlī ensured was obeyed completely. He ordered ʿAlī to break all the idols in the Kaaba and to purify the shrine after its defilement by the polytheism of the pre-Islamic era, which Muslims call al-jāhiliyyah (“the age of ignorance”). ʿAlī also was charged with settling several disputes and putting down the uprisings of various tribes.

At Ghadīr Khumm in 632, while returning to Medina from his last pilgrimage, the Prophet made certain statements about ʿAlī that have been interpreted in very different ways by Sunnis and Shiʿis. According to both traditions, Muhammad said that ʿAlī was his inheritor and brother and that whoever accepted the Prophet as his mawlā (“master” or “trusted friend” but also, contradictorily, “client” or “protegé”) also should accept ʿAlī as his mawlā. The Shiʿah regard these statements as constituting the investiture of ʿAlī as the successor of the Prophet and as the first imam. The Sunnis, by contrast, take them only as an expression of the Prophet’s closeness to ʿAlī and of his wish that ʿAlī, as his cousin and son-in-law, inherit his family responsibilities upon his death. Many later Islamic Sufis and esoterists also interpret the episode as the transfer of the Prophet’s spiritual power and authority to ʿAlī (mawlā is related to wīlāyah or walāyah, meaning “rule,” “initiation,” “spiritual authority,” or “power”), whom they regard as the walī (literally “friend,” usually translated as “saint”) par excellence.