The Old Testament Scriptures were classified into three sections (Law, Prophets and Psalms) by the Lord Jesus Christ. It is interesting to observe how the author of the Epistle to the Hebrews ascribes their authorship to the Holy Spirit of God.

"And He said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the Law of Moses, and in the Prophets, and in the Psalms, concerning Me." Luke 24:44

Law of Moses (Exodus 30:10)
"The Holy Ghost this signifying, that the way into the holiest of all was not yet made manifest, while as the first tabernacle was yet standing" Hebrews 9:8

Prophets (Jeremiah 31:33-34)
"Whereof the Holy Ghost also is a witness to us: for after that He had said before, this is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them; and their sins and iniquities will I remember no more." Hebrews 10:15-17

Psalms (Psalm 95:7-8)
"Wherefore as the Holy Ghost saith, To day if ye will hear His voice, harden not your hearts, as in the provocation, in the day of temptation in the wilderness" Hebrews 3:7-8