The Annals of King Sejong, Vol 102 :

[The entry for the twelfth month of the twenty-fifth year of Sejong’s reign]

"This month, His Highness has personally created the twenty-eight letters of the Vernacular Script . . . ."

(Translated by Ledyard [1966], quoted in Ramsey [2006: 22]):

Preface of the Hunmin chŏngŭm:

"The sounds of our country’s language are different from those of the Middle Kingdom (i.e., China) and are not confluent with the sounds of characters. Therefore, among the ignorant people, there have been many who, having something they want to put into words, have in the end been unable to express their feelings. I have been distressed because of this, and have newly designed twenty-eight letters, which I wish to have everyone practice at their ease and make convenient for their daily use." (Translated by Ledyard [1966], quoted in Ramsey [2006: 26])

Gedyard, Gari. 1966. The Korean Language Reform of 1446: the origin, background, and early history of the Korean Alphabet. Ph. D dissertation, University of California, Berkeley.