John Gill Reformed Baptist @doctorgill

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...

Geneva Bible Notes Reformed @genevanotes

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

Adam Clarke Methodist @methodicalclarke

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.