Abul Qassim Hasan bin Farah bin Hawshab, generally was known by the epithet, Mansur al-Yamen (the victorious one of the Yamen). Ibn Hawshab was originally a Twelver, and is said to have spent most of time in a secluded spot on the bank of Euphrates. In such state, he is reported to have met Imam Radi Abdullah and discoursed with him on religion. Imam left him after promising to see him again. Ibn Hawshab was impressed by his chance meeting, and was eager to further meeting. After an anxious wait for several days, the Imam did not appear again, thus he became restless and began to search him. Despite his frantic efforts to locate the Imam's whereabouts, he could not trace him. After sometimes, he accidentally met the Imam's deputy, and through him, Ibn Hawshab eventually succeeded in reaching the Imam's presence. Imam Radi Abdullah answered his queries to his satisfaction and assuaged his doubts, and at length he espoused Ismailism.

When the Imam found that Ibn Hawshab was firmly grounded in Ismaili faith and groomed enough for the responsibility of its promulgation, he jointly entrusted him and his colleague, Ibn Fazal, with the task of Ismaili mission in Yamen. Before they set off on their venture, the Imam summoned them in a private audience and urged to respect each other, and avoid any sort of religious difference. He also entrusted Ibn Hawshab with a voluminous tome which comprehensively dwelled upon the exoteric and esoteric aspects of Ismaili faith. Thus, being equipped with verbal as well as written guidances, both of them set forth on their mission to Yamen in 266/880. Mecca was their first destination, and accosted the pilgrim caravan from Yamen. They proceeded at last to Yamen, and after reaching, both of them separated. Ibn Hawshab headed towards the southern region, and focused his mind on the village of Adanla'a, thickly populated by the Shiites. He married to a local woman and settled down in Adanla'a under strict taqiya. He succeeded to convert the inhabitants. When he found the time appropriate to reveal his identity, he discreetly started his mission, inviting the people to the Ismaili fold and accepting oath of allegiance on behalf of Imam Radi Abdullah and his successor. On other side, Ibn Fazal also succeeded in winning the sympathy and adherence of the people of Saroyafoa.

Ibn Hawshab had managed to take possession of a stronghold constructed on a hillock and made it his headquarters. He arranged military training for his followers. He also took possession of Jabal al-Jusaysah and Jabal al-Maswar. Ibn Hawshab however assured the people that his campaigns were neither after booty nor personal glory, but these were meant to promulgate true Islamic message through Ismailism. Finally, he conquered Sana'a, the capital of Yamen, and exiled the ruling tribe of Banu Laydir, and established Ismaili authority in Yamen.

The Ismaili mission reached the apex of its influence in Yamen, from where Ibn Hawshab dispatched many da'is to the farthest corners. Thus, Yamen became a vital zone and an important hub of Ismaili dawa. In the time of Imam Radi Abdullah, Abu Abdullah al-Shi'i had embraced Ismailism, who was sent to Yamen for further training. Ibn Hawshab was loyal to the Imam till his last breath and died in 302/914. While reading his biography, we will greatly move the streak of intellectual honesty which ran through his very nature.