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Al-Farq bayn al-Firaq is a book by the Shafi'i scholar Abu Mansur al-Baghdadi (d1037 CE) outlining the doctrinal positions of various sects and schisms in Islam. Written as an explanation of the hadith regarding the division of the Muslim ummah into 73 sects, the book explains the hadith, lays out the various beliefs of 72 misguided sects and ends by explaining the beliefs of Orthodox Sunni Islam. In his Usul al-Din (Basics of Religion) he puts that: the people of the lands of al-Yaman upon the Zanj coastline (Zanzibar) were all from the Ahl ul-Hadeeth. (= Conquered by the companions of the Prophet). No other author is known to have made this statement.
Full name: Abu Mansur 'Abd al-Kahir al-Baghdadi
Taken from: Die Richtungen der islamischen Koranauslegung By Ignaz GoldziherMost Mu' taziliten (1) state that the Zindsch, Turken and Chazaren (2) are able to learn the (rhetorical) art of the Koran and to bring something similar and even still more splendid; only the ability lacks them to bring the things into the correct order.
Taken from: Shorter encyclopaedia of Islam by Sir Hamilton Alexander Rosskeen Gibb, Johannes Hendrik Kramers,
The followers of Babak (Iranian-Zoroastrian freedom fighter against the Arabs) make the founder of their religion a prince of theirs who lived in pre- Islamic times, called Sharwin Whose father was of the Zandj whereas his mother was the daughter of a Persian king.
Usul al-Din (Basics of Religion)
Taken from: Shaykh Ahmad ibn Muhammad, A History of the People of Hadeeth
Full name: Abd al-Kahir b. Tahir, Abu Mansur al-Shafi`I al-Baghdadi
It is clear that the people of the lands of ar-Room (7), al-Jazeerah (8), ash-Shaam (9), Adharbayjaan (Azerbaijan), Baabul-Abwaab (3) and others which were conquered were all upon the madhhab of
the Ahl ul-Hadeeth. Also the inhabitants of the lands of Ifreeqiyyah (4), Andalus (5) and all the countries behind the Western Sea, were from the Ahl ul-Hadeeth (6). Also the people of the lands
of al-Yaman upon the Zanj coastline (Zanzibar) were all from the Ahl ul-Hadeeth.
(1) Mu' taziliten: : Originally; Those who would neither condemn nor sanction Caliph ʿAlī or his opponents but took a middle position. Later: By the 10th century CE the term had also come to refer to an Islamic school of speculative theology in Basra and Baghdad.
(2) Chazaren; Khazars: Semi nomadic people from south Russia, south Ukraine, Crimea, Kazakhstan.
(3) Baabul-Abwaab: Babal Abwab; Ancient city in Dagestan on the western shore of the Caspian Sea.
(4) Ifreeqiyyah: here Ifriqaya: coastal parts of eastern Algeria, Tunisia and western Libya.
(5) Andalus: Muslim Spain.
(6) All the people the companions of the Prophet conquered were called: Ahl ul-Hadeeth.
(7) Rome
(8) meaning the island: the land between the Tigris and the Euphrates.
(9) Syria