Quote extracted from this link:

“…W. W. Phelps presented an article (not a revelation) on “Marriage,” and Oliver Cowdery one on “Government and Laws in General,” both of which were ordered printed in the book of “Doctrine and Covenants.” There was also printed in the book a series of Seven Lectures on Faith, that had previously been delivered before a theological class in Kirtland. So that the Doctrine and Covenants then comprised the Articles of Faith, seven in number, the two Articles on Marriage and Government and a collection of Revelations, (not all that had been given, by the way,) the last forming the body and greater part of the book.” (See History of the Church, Vol. II, chapter xviii.)

[According to B. H. Roberts] History of the Church, Vol. 2, p. 246, Footnotes.