WCF:
Ch 7
VI. Under the gospel, when Christ the substance was exhibited, the ordinances in which this covenant is dispensed, are the preaching of the Word, and the administration of the sacraments of Baptism and the Lord’s Supper; which, though fewer in number, and administered with more simplicity and less outward glory, yet in them it is held forth in more fullness, evidence, and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the New Testament. There are not, therefore, two covenants of grace differing in substance, but one and the same under various dispensations.
The above links preaching with the distribution of the sacraments
Ch 21
V. The reading of the Scriptures with godly fear; the sound preaching, and conscionable hearing of the Word, in obedience unto God with understanding, faith, and reverence; singing of psalms with grace in the heart; as, also, the due administration and worthy receiving of the sacraments instituted by Christ; are all parts of the ordinary religious worship of God: besides religious oaths, and vows, solemn fastings, and thanksgivings upon special occasion; which are, in their several times and seasons, to be used in an holy and religious manner.
The above links preaching w/ 'due administration'.
Ch 23
III. The civil magistrate may not assume to himself the administration of the Word and sacraments, or the power of the keys of the kingdom of heaven: yet he hath authority, and it is his duty, to take order, that unity and peace be preserved in the Church, that the truth of God be kept pure and entire; that all blasphemies and heresies be suppressed; all corruptions and abuses in worship and discipline prevented or reformed; and all the ordinances of God duly settled, administered, and observed. For the better effecting whereof, he hath power to call synods, to be present at them, and to provide that whatsoever is transacted in them be according to the mind of God.
The above condemns a non-ordained man to assume 'to himself the administration of the word and sacraments'. This would include preaching.
Ch 27
IV. There be only two sacraments ordained by Christ our Lord in the gospels, that is to say, Baptism and the Supper of the Lord: neither or which may be dispensed by any but a minister of the Word, lawfully ordained.
'Minister of the word' says much. The distinction between a 'minister of the word' and one who is not a 'minister of the word' must be considered. If I read the word to my family, I am not administering the M of G, nor am I preaching to them. I am exhorting, witnessing, educating as a parent is called to do.
Ch 28
II. The outward element to be used in the sacrament is water, wherewith the party is to be baptized in the name of the Father, and of the Son, and of the Holy Ghost, by a minister of the gospel, lawfully called thereunto.
Since the sacraments are a means of grace and can only be distributed by an ordained man, 'called thereunto', it would follow that preaching falls under the same criteria.
From the Directory for the Publick worship of God:
Of the Preaching of the Word.
PREACHING of the word, being the power of God unto salvation, and one of the greatest and most excellent works belonging to the ministry of the gospel, should be so performed, that the workman need not be ashamed, but may save himself, and those that hear him.
It is presupposed, (according to the rules for ordination,) that the minister of Christ is in some good measure gifted for so weighty a service, by his skill in the original languages, and in such arts and sciences as are handmaids unto divinity; by his knowledge in the whole body of theology, but most of all in the holy scriptures, having his senses and heart exercised in them above the common sort of believers; and by the illumination of God’s Spirit, and other gifts of edification, which (together with reading and studying of the word) he ought still to seek by prayer, and an humble heart, resolving to admit and receive any truth not yet attained, whenever God shall make it known unto him. All which he is to make use of, and improve, in his private preparations, before he deliver in public what he hath provided.