I think that would be year 607, in which Sui emperor Yang Guang set up the examinations bureau. You have the original source in Zizhi Tongjian, scroll 180: 。改上柱國以下官為大夫;置殿內省,與尚書、門下、內史、秘書為五省; - the examinations bureau would be 門下省.
Perhaps this can help you as a starting point to find an appropriate citation in modern works.
Though I would appreciate it if somebody more familiar than I am with Chinese history could correct me if necessary.
I found the following article by Denis Twitchett useful: "The Birth of the Chinese Meritocracy: Bureaucrats and Examinations in T'ang China." (Lecture delivered to the China Society in London on 17th December 1974). China Society Occasional Papers 18 (1976), 1-33. Twitchett emphasizes the gradual implementation of the examination system which for some time existed side-by-side with other forms of recruitment of officials.
Yuri Pines and Benjamin Elman, who I consider authorities on the subject, have chapters regarding this in the recently published "East Asian Challenge to Democracy: Political Meritocracy in Comparative Perspective" (Cambridge University Press, 2013).