The Spirit of the Lord spake by me - Hence the matter of his writing came by direct and immediate inspiration.
His word was in my tongue - Hence the words of this writing were as directly inspired as the matter.

The Spirit of the LORD spake by me, and his word [was] in my (b) tongue.
(b) Meaning, he spoke nothing but by the motion of God's Spirit.

The Spirit of the Lord spake by me,.... The psalms and songs he composed were not the fruits of his own genius, but were written by him under the inspiration of the Spirit of God; by whom holy men of God, the penmen of the Scriptures, spoke, even as they were moved by the Holy Ghost, of whom David was one, being a prophet; see Acts 1:16 Acts 2:30; so the Targum here,"David spoke by the spirit of prophecy of the Lord:''or spake "in me" (h); what he spoke was first internally impressed upon his mind by the Spirit of God, and then he expressed it with his tongue, as follows:
and his word was in my tongue; not only the matter of his psalms was indited by the Spirit of God, and suggested to his mind; but the very words in which they are delivered were given to him, and he was directed to make use of them, and did.
(h) "in me", Montanus, Junius & Tremellius.

The Spirit of the Lord spake by me--Nothing can more clearly show that all that is excellent in spirit, beautiful in language, or grand in prophetic imagery, which the Psalm of David contain, were owing, not to his superiority in natural talents or acquired knowledge, but to the suggestion and dictates of God's Spirit.

His word - The following words, and consequently the other words and Psalm composed and uttered by me upon the like solemn occasions, are not to be looked upon as human inventions, but both the matter and the words of them are suggested by God's spirit, the great teacher of the church.