Shaah Waleeullaah Delhwi said, “know that the people were not agreed upon Taqleed with reference to one particular madhab before the fourth century” (Hujjatullah al-Baaligha pg.157)
Shah Abdul Azeez said “the people were not doing Taqleed of one madhab up until the time of Imaam Maalik” (Baustan al-Muhadditheen pg.17)
Imaam Ibn Qayyim said,”this innovation of Taqleed started in the fourth century and the Messenger of Allaah censured it verbally” (Alaam al-Muwaqqieen 1/222)
Ahlul hadeeth - The Way Of The Salaf 