Abu'l-Qāsim al-Ḥasan ibn Faraj ibn Ḥawshab ibn Zādān al-Najjār al-Kūfī (Arabic: أبو القاسم الحسن ابن فرج بن حوشب زاذان النجار الكوفي; died 31 December 914), better known simply as Ibn Ḥawshab, or by his honorific of Manṣūr al-Yaman (Arabic: منصور اليمن, lit. 'Conqueror of Yemen'), was a senior Isma'ili missionary (dāʿī) from the environs of Kufa. In cooperation with Ali ibn al-Fadl al-Jayshani, he established the Isma'ili creed in Yemen and conquered much of that country in the 890s and 900s in the name of the Isma'ili imam, Abdallah al-Mahdi, who at the time was still in hiding. After al-Mahdi proclaimed himself publicly in Ifriqiya in 909 and established the Fatimid Caliphate, Ibn al-Fadl turned against him and forced Ibn Hawshab to a subordinate position. Ibn Hawshab's life is known from an autobiography he wrote, while later Isma'ili tradition ascribes two theological treatises to him.

Ibn Hawshab was born at a village near the Nahr Nars canal, in the environs of Kufa in southern Iraq. His origin is unknown, although later Isma'ili tradition held that he was a descendant of Muslim ibn Aqil ibn Abi Talib (a nephew of Ali ibn Abi Talib).

Sources differ on his profession, portraying him as a linen weaver or a carpenter. He hailed from a family that were adherents of Twelver Shi'ism. According to his own report, he had been experiencing a crisis of faith after the death of the eleventh imam, Hasan al-Askari, in 874, apparently without male progeny. Eventually, the Twelvers came to believe in an infant son of al-Askari as the twelfth and hidden imam (whence the name "Twelvers"), who would one day return as the mahdī, the messianic figure of Islamic eschatology, who according to legend would overthrow the usurping Abbasid caliphs and destroy their capital Baghdad, restore the unity of the Muslims, conquer Constantinople, ensure the final triumph of Islam and establish a reign of peace and justice. However, that belief was not yet firmly established during the early years after Hasan al-Askari's death. Like Ibn Hawshab, many Shi'ites had doubts about the claims made about the twelfth imam, and were further demoralized by the political impotence and quietism of the Twelver leadership. In this climate, the millennialism of the Isma'ilis, who preached the imminent return of a mahdī, and the start of a new messianic era of justice and the revelation of the true religion, was very attractive to dissatisfied Twelvers.

According to his own account, Ibn Hawshab was converted to the rival Isma'ili branch of Shi'ism by an old man who came to him while he was studying the Quran at the bank of the Euphrates. Pro-Fatimid accounts hold that the agent (dāʿī) in question was Firuz, who was chief dāʿī at the movement's headquarters at Salamiya and the chief proxy (bāb, "gate") for the hidden Isma'ili imam, whereas the anti-Fatimid Qarmatian tradition holds that this was Ibn Abi'l-Fawaris, a lieutenant of Abdan, the chief dāʿī of Iraq.

Shortly after, Ibn Hawshab claims that he met the Isma'ili imam, then secretly living at Salamiya. After his training was complete, he was tasked with spreading the Isma'ili creed to Yemen. He was joined by a recently converted native Yemeni, Ali ibn al-Fadl al-Jayshani, and set off in late May or early June 881.

The two missionaries made for Kufa, where they joined the pilgrim caravans, whose multitudes, gathered from all corners of the Islamic world, allowed them to travel with anonymity. After completing the rituals of the pilgrimage at Mecca, the two men arrived in northern Yemen in August 881. The Yemen was at the time a troubled province of the Abbasid empire. Caliphal authority had traditionally been weak and mostly limited to the capital, Sana'a, while in the rest of the country tribal conflicts, sometimes dating to pre-Islamic times, persisted. At the time of Ibn Hawshab and Ibn al-Fadl's arrival, the country was politically fragmented and only loosely under Abbasid suzerainty. Much of the interior was held by the Yu'firid dynasty, who as Sunnis recognized the Abbasids. After capturing Sana'a in 861, their rule extended from Sa'ada in the north to al-Janad [ar] (northeast of Taiz) in the south and Hadramawt in the east. A rival dynasty, the Ziyadids, also nominally loyal to the Abbasids, held Zabid on the western coastal plain, and at times exercised significant control over wide portions of the country. The Manakhi family ruled the southern highlands around Taiz, while the northern parts of the country were in practice dominated by warring tribes owing allegiance to no-one. The lack of political unity, the remoteness of the province and its inaccessible terrain, along with deep-rooted Shi'a sympathies in the local population, made Yemen "manifestly fertile territory for any charismatic leader equipped with tenacity and political acumen to realise his ambitions".

After travelling through Sana'a and al-Janad, Ibn Hawshab stayed for a while in Aden, where he passed himself off as a cotton merchant. Ibn Hawshab was evidently the senior of the two, but at some point, Ali ibn al-Fadl left him, moving to his home town of Jayshan (near modern Qa'tabah [ar]), where he independently began his mission in the mountains of Jebel Yafi'i. Ibn Hawshab does not appear to have had much success in gaining converts in Aden. When he met some pro-Shi'a members of the northern Banu Musa clan, who were open to his teachings and invited him to join them in their homeland, he left Aden and settled in the village of Adan La'a, west of Sana'a. There Ibn Hawshab settled in the house of a Shi'a partisan who had died in the Yu'firid dungeons, married his orphaned daughter, and in 883/4 began his public mission (daʿwa), proclaiming the imminent appearance of the mahdī.

As in other areas of the Islamic world, this call soon attracted many followers. The widespread millennialist expectations of the period coincided with a deep crisis of the Abbasid Caliphate (the Anarchy at Samarra, followed by the Zanj Rebellion), and with dissatisfaction among many Twelver adherents, to enhance the appeal of the revolutionary Isma'ili message. Ibn Hawshab quickly made many converts, with his wife's family foremost among them: one of her cousins, al-Haytham, was sent as a dāʿī to Sindh, thus beginning a long history of Isma'ili presence in the Indian subcontinent. Furthermore, Abdallah ibn al-Abbas al-Shawiri was sent to Egypt; Abu Zakariyya al-Tamami to Bahrayn; and others to Yamama and parts of India (most probably Gujarat). Most consequential among the dāʿīs trained and sent by Ibn Hawshab was Abu Abdallah al-Shi'i, a native of Sana'a. On Ibn Hawshab's instructions, in 893 he left for the Maghreb, where he began proselytizing among the Kutama Berbers. His mission was extremely successful. Backed by the Kutama, in 903 he was able to rise in revolt against the Aghlabid emirs of Ifriqiya, culminating in their overthrow and the establishment of the Fatimid Caliphate in 909.