Abūʾl-Ḥusayn Yaḥyā ibn al-Ḥusayn ibn al-Qāsim ibn Ibrāhīm Ṭabāṭabā al-Ḥasanī (al-Rass/Medina, 859 – Sa'dah, 18 August 911), better known by his honorific title of al-Hādī ilāʾl-Ḥaqq (Arabic: الهادي الى الحق, lit. 'the Guide to the Truth'), was a religious and political leader in the Arabian Peninsula. He was the first Zaydi imam who ruled portions of Yemen from 897 to 911. He is also the ancestor of the Rassid dynasty which ruled Yemen intermittently until the North Yemen Civil War in 1962.
According to the later Zaydi sources, Yahya ibn al-Husayn was born in Medina in 859. However, it appears that he was actually born at a village (likely modern al-Dur or Dur Abi al-Qasim, some 57 kilometres (35 mi) southwest of Medina) near the wadi al-Rass, where his grandfather, al-Qasim "al-Rassi", had settled after bringing his family over from Egypt around 827. He was on both sides of his family a descendant of al-Hasan, a son of Ali ibn Abu Talib, the son-in-law of Muhammad and first Shi'a imam: his father al-Husayn was a great-great-grandson of al-Hasan's grandson Ibrahim al-Shibh, while his mother, Umm al-Hasan Fatima, was a great-great-granddaughter of al-Hasan's grandson Da'ud.
Yahya was born into a particularly prominent branch of the Hasanid line. His grandfather, al-Qasim al-Rassi, was one of the chief authorities of the Zaydi school of Shi'a Islam, and was honoured as "Star of the Family of the Prophet of God" (Najm Āl Rasūl Allāh) and "Interpreter of the Faith" (Turjumān al-Dīn). Al-Qasim's brother, Ibn Tabataba, raised a rebellion against the Abbasid Caliphate at Kufa in 814. Yahya's father, al-Husayn, was of lesser status, but was accounted a learned man and esteemed as a reliable transmitter of hadiths.
Yahya married his paternal first cousin, Fatima. The couple had nine sons: Muhammad, Ahmad, al-Hasan, Yahya, Isma'il, al-Husayn, Abdallah, Ja'far, and Isa. Both Muhammad and Ahmad would succeed their father as imams, and most of the imams of Yemen descend from Ahmad. Of the 73 imams of Yemen who followed Yahya, 60 were his direct descendants, and six of the rest were descended from his brother, Abdallah, and his uncle, Muhammad.
Already at a very young age, Yahya distinguished himself for his character and intelligence: he was strong, brave, and well versed in Islamic jurisprudence (fiqh), to the extent that at the age of seventeen he could compose treatises and issue judgments. As a result, his family quickly came to regard him as a suitable candidate for the imamate. In Zaydi doctrine, the imam has to be a "Fatimid", i.e. a descendant of Muhammad's daughter Fatimah and her sons, al-Hasan and al-Husayn, but the position is not hereditary or by appointment (naṣṣ), unlike in the Twelver and Isma'ili traditions of Shi'a Islam. Instead, it can be claimed by any qualified Fatimid who fulfills a number (usually 14) of stringent conditions (religious learning, piety, bravery, etc.), by "rising" (khurūj) and "calling" (daʿwa) for the allegiance of the faithful. Zaydi doctrine emphasized that the imamate was not contingent on popular acclaim or election; the very act of daʿwa denotes God's choice. On the other hand, if a more excellent candidate appears, the incumbent imam is obliged to acknowledge him. Both al-Qasim al-Rassi, and Yahya's maternal great-grandfather, Muhammad ibn Sulayman ibn Da'ud, are considered as imams by the Zaydis. Later Zaydi tradition associated prophecies about Yahya's eventual imamate, both by his grandfather al-Qasim, as well as by the Islamic prophet Muhammad himself.
Like all Shi'a imams, Yahya was opposed to the Abbasid Caliphate, but was also well aware of the failure of all past attempts by Shi'a candidates to stage a successful military uprising against the Abbasids, most notably the uprising of Muhammad al-Nafs al-Zakiyya in 762, the revolt of al-Husayn ibn Ali al-Abid in 786, and a number of failed Zaydi risings in the early 9th century. These failures had obliged his grandfather to abstain from proclaiming his daʿwa in public, even though his claim to the imamate had been recognized by various communities from the Hejaz, Iraq and Persia, who had sent him pledges of allegiance. The common factor of these revolts was that they had happened in the heartlands of the Islamic world—Iraq and the Hejaz—typically beginning with a public proclamation at Medina or Mecca. In marked contrast to their failures, anti-Abbasid movements on the periphery of the caliphate were more successful: Idris ibn Abdallah, fleeing from the suppression of the revolt of al-Husayn ibn Ali al-Abid, had founded a Zaydi state in what is now Morocco, while a distant relative of Yahya's, Hasan ibn Zayd, had founded a Zaydi state in Tabaristan, a mountainous region on the southern shores of the Caspian Sea.
Between 884 and 889, Yahya and his family visited Tabaristan, then ruled by Muhammad ibn Zayd, a brother of the founder of the Zaydi state there. The family settled at Amol, the local capital, while the Zaydi imams were at the eastern region of Jurjan. Yahya quickly attracted attention, as his own uncles and cousins took to proclaiming him as the candidate for the imamate. These stories suggest that Yahya may have contemplated declaring his daʿwa at Amol. At any rate, the rumours surrounding him aroused the suspicions of Muhammad ibn Zayd, who was already ruling as imam. A khurūj by Yahya would inevitably be a challenge to Muhammad to relinquish his authority, and the latter's expected refusal would lead to an armed clash between the supporters of the two rivals. Yahya assured Muhammad that he had no such plans, but he soon learned of a plot to have him and his family arrested, forcing them to abandon Tabaristan in haste. The family returned to the Hejaz, but Yahya appears to have also briefly visited the Abbasid capital, Baghdad.
In 893/4, Yahya for the first time journeyed to the Yemen, on the invitation of the powerful Banu Futaym clan, living in the northern region of Sa'dah, in order to settle their disputes. 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 Yahya'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 overlordship of the Abbasids. After capturing Sana'a in 861, their rule extended from Sa'dah in the north to al-Janad (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 interior of the country. The Banu Manakh 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". Furthermore, the distance to the other Zaydi state at Tabaristan meant that here, Yahya would not have to contend with a rival Zaydi imam.
Yahya's first attempt was cut short. He reached al-Sharafah, some distance from Sana'a, but was then forced to turn back due to indiscipline among his own men, and returned to al-Fara, southwest of Medina. A new opportunity offered itself three years later, when the same tribal leaders invited Yahya to come back and end the strife-torn conditions of northern Yemen. He arrived in Sa'dah on 15 March 897. Shortly after, he proclaimed his daʿwa and assumed the title of commander of the faithful, with the regnal name of al-Hādī ilāʾl-Ḥaqq ("the Guide to the Truth"), or al-Hadi for short.