1 Pt 1:24-25, Mt 24:35, Lk 21:33
In the Old Testament, Isaiah said that the word of God would stand forever. (Isa 40:6-8)
Isa 40:6 The voice said, Cry. And he said, What shall I cry? All flesh is grass, and all the goodliness thereof is as the flower of the field: 7 The grass withereth, the flower fadeth: because the spirit of the Lord bloweth upon it: surely the people is grass. 8 The grass withereth, the flower fadeth: but the word of our God shall stand for ever.
Jesus made it clear that his words are eternal. (Matthew 24:35, Luke 21:33)
Matthew 24:35 Heaven and earth shall pass away, but my words shall not pass away.
Luke 21:33 Heaven and earth shall pass away: but my words shall not pass away.
In the Old Testament, the eternal Word of God spoken of by Isaiah is this gospel, that is, Jesus is the Christ. (1 Peter 1:24-25)
1 Peter 1:24 For all flesh is as grass, and all the glory of man as the flower of grass. The grass withereth, and the flower thereof falleth away: 25 But the word of the Lord endureth for ever. And this is the word which by the gospel is preached unto you.