An Outline of the History of Restriction on Ijtihad
Answers to the issues pertinent to this question become obvious on referring to many books of the Ahl al-Sunnah-some of which we shall mention by the way of a general guidance regarding sources-where these answers can be found and from which we shall be giving citations.
Among these books is al-Mawa'iz wa al-i'tibar fi al-khutat wa al-athar, a comprehensive popular work on Egyptian history, by al-Shaykh Taqi al-Din Abu al-'Abbas Ahmad ibn Ali ibn 'Abd al-Qadir ibn Muhammad al-Bali al-Qahiri, popularly known as al-Maqrizi, a nisbah relating to a quarter of Ba'labak known as al-Maqarizah, where he was born in 766/1364 and died in Cairo in 845/1441. It has been published a number of times, complete and in parts, and has been translated into French. It is relied upon and accepted by all later authors who cite what he mentions as an established fact. In this book (vol. 4, p. 141 ff.) al-Maqrizi has dealt elaborately with differences among the schools of law, devoting several pages to the topic under the heading "Dhikr madhahib ahl Misr wa nihalihim mundhu iftataha 'Umar ibn al-'As ila an saru ila i'tiqad al-madhahib al-arba'ah" ('Discussion of the legal sects and creeds of the people of Egypt from the time of 'Umar ibn al-'As' conquest up to their acceptance of the four schools of law'). 
Another source is the history of al-Ya'qubi, Ahmad ibn Abi Ya'qub al-Baghdadi (d. after 296/908), and printed in Najaf in 1358 H.
Another source is al-Hawadith al-jami'ah fi al-mi'at al-sabi'ah of Kamal al-Din 'Abd al-Razzaq ibn al-Marwazi al-Fuwati al-Baghdadi (d. 723/1323), printed in Baghdad at Matba'at al-Furat in 1351 H. It mentions some points to which we will refer at an appropriate place.
Among the sources are al-Insaf fi bayan sabab al-ikhtilaf and 'Aqd al-jid fi ahkam al-ijtihad wa al-taqlid. These two are the works of Shah Wali Allah Ahmad ibn 'Abd al-Rahim al-'Umari al-Dehlawi (b. 1114/1702; d. 1180/1766 .or 1176/1762). Both have been printed together with al-Muqabasat of Abu Hayyan al-Tawhidi, as well as with Kashf al-zur wa al-buhtan. 
Other sources are al-Iqlid li adillat al-ijtihad wa al-taqlid and al-Tariqat al-muthla fi al-isharah ila tark al-taqlid, both by Siddiq Hasan Khan al-Qannawji al-Bukhari (d. 1308/1890), both printed at Astanah in 1295 and 1296 H. There is another book of his called Husul al-ma'mul min 'ilm al-usul, which has been published in al-Jawa'ib in 1296 H. its "almaqsad al-thalith" (sic)  deals with ijtihad and taqlid and is the last of three books published together in one volume, the first two being Luqtat al-ajlan fi ma tamussu ila ma'rifathihi hajat al-insan and Khabi'at al-akwan fi iftiraq al-umam 'ala al-madhahib wa al-adyan.
There is also the article of the author of al-Sa'adah, Ahmad Taymur Basha ibn Isma'il ibn Muhammad (b. 1288/1871 at Cairo), entitled "Nazrah ta'rikhiyyah fi huduth al- madhahib al-arba'ah," published in the magazine al-Zahra' at the beginning of its second year and later printed separately in Cairo by Muhibb al-Din al-Khatib on 15th Rajab 1344 H. He has described in this article the causes of the birth of the four schools of law, the places where they initially emerged, the mode of their growth and spread to other points, and a brief historical account of the extinct schools from the time of their emergence, their gradual spread and the extent of their currency until their eventual extinction.
The most detailed study of this issue among the sources mentioned is that of the learned Muhammad Farid Wajdi Beg (b. 1293/1876) in the third volume of the Da'irat al-ma'arif fi al-qarn al-rabi' 'ashar al-hijri,  published in 1330 H. His discussion on the subject, under the root j-h-d, covers sixty pages. There he reproduces the two books mentioned earlier in their entirety, i.e. al-Insaf fi bayan sabab al-ikhtilaf and 'Iqd al-jid. He has also discussed the issue at length under the root dh-h-b, referring here to what he has already mentioned under j-h-d.
These are the books that were accessible to me on this subject, and Siddiq Hasan Khan, in the third "maqsad"  of his book Husul al-ma'mul, mentions some other works in this regard asking the reader to refer to them. Of these are:
Adab al-talab wa muntaha al-irab by al-Shawkani,