Thank you, my dear friend, for this interesting question! Let’s read the first two verses in the Bible, Genesis 1:1-2 (New Translation): “In the beginning God created the heavens and the earth. And the earth was waste and empty, and darkness was on the face of the deep, and the Spirit of God was hovering over the face of the waters.” Thus, we see that already in Genesis 1:2 that the Holy Spirit was identified in the Bible. But to be honest, I believe that we can assume the presence of the Holy Spirit as early as in verse 1 as we read that “in the beginning, God created the heavens and the earth…”.  The Hebrew word for God here is “ʼĕlôhîym” which is defined as “gods in the ordinary sense; but specifically used (in the plural thus, especially with the article) of the supreme God.”

We know from Scripture that the Holy Spirit is one of the persons of the Trinity, along with the Father and the Son. All had their part in creation. Now, how do I know this? Well, in Genesis 1:26 you’ll note the plural form of “God”, “And God said, Let us make man in OUR image, after OUR likeness…”  Following up on this thought, you might consider the first 3 verses of John chapter 1, “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things received being through him, and without him not one thing received being which has received being.” We know from John 1:14 that the “Word” in John 1 is in fact Christ, God the Son, and thus He along with the Holy Spirit were One with God from the beginning (Genesis 1:2), and indeed, they were identified in the Godhead.  We read in Colossians 1:16-17, “For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him: And he is before all things, and by him all things consist.”

So then, I think it is clear that the earliest specific mention of the Holy Spirit in the Bible would be Genesis 1:2.

To learn more about the existence of the Holy Trinity, please click on this Answers From the Book link:  https://answersfromthebook.net/can-you-explain-the-trinity/  (SF)  (670.6)