Abu al-Husayn Ahmad ibn Abd Allah ibn Muhammad ibn Isma'il (Arabic: أَبُو ٱلْحُسَيْن أَحْمَد ٱبْن عَبْد ٱللَّٰه ٱبْن مُحَمَّد ٱبْن إسْماعِيل, romanized: Abū al-Ḥusayn Aḥmad ibn ʿAbd Allāh ibn Muḥammad ibn Ismāʿīl; c. 790–840), commonly known as Muhammad al-Taqi (Arabic: مُحَمَّد ٱلْتَقِيّ, romanized: Muḥammad al-Taqī, lit. 'Muhammad the Pious'), was a descendant of the Islamic prophet Muhammad and the ninth of the Isma'ili Imams, succeeding his father, Ahmad al-Wafi (d. 828). Like his father, he lived primarily in Salamiyah, and Abd Allah ibn Maymun al-Qaddah, the chief missionary (da'i), continued to serve as the hijab (lit. 'cover') for him. Known by the title Ṣāḥib al-Rasāʾil (lit. 'lord of the epistles'), al-Taqi is said to have prepared with his followers an encyclopedic text called the Encyclopedia of the Brethren of Purity (Rasāʾil Ikhwān al-Ṣafā). He died in 840 in Salamiyah and was succeeded by his son al-Husayn.
With the death of Ja'far al-Sadiq in 148/765, Isma'il (d. 158/775) and Muhammad (d. 197/813), the gravity of persecutions of the Abbasids had considerably increased. The Isma'ili Imams were impelled to thicken their hiding, therefore, the first dawr al-satr came into force from 197/813 to 268/882, wherein the Imams were known as al-a'imma al-masturin (lit. 'the concealed Imams'). The concealment ended with the establishment of the Fatimid Caliphate (r. 909–1171).
