Heb 1:10-12, 1 Pt 1:24-25, Mt 24:35, Lk 21:33
In the Old Testament, God said that heaven and earth would pass away, but God's salvation and His righteousness are eternal. (Isa 51:6)
Isa 51:6 Lift up your eyes to the heavens, and look upon the earth beneath: for the heavens shall vanish away like smoke, and the earth shall wax old like a garment, and they that dwell therein shall die in like manner: but my salvation shall be for ever, and my righteousness shall not be abolished.
The writer of Hebrews quoted the Old Testament saying that God is eternal. (Heb 1:10-12)
Heb 1:10 And, Thou, Lord, in the beginning hast laid the foundation of the earth; and the heavens are the works of thine hands: 11 They shall perish; but thou remainest; and they all shall wax old as doth a garment; 12 And as a vesture shalt thou fold them up, and they shall be changed: but thou art the same, and thy years shall not fail.
Jesus said his own 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.
The eternal word of the Lord prophesied in the Old Testament is the gospel that 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.