Isa 40:8, Mt 24:34-35, 1 Pt 1:25
The Old Testament says that the Word of God is eternal. (Psalms 119:89, Isaiah 40:8)
Psalms 119:89 For ever, O Lord, thy word is settled in heaven.
Isaiah 40:8 The grass withereth, the flower fadeth: but the word of our God shall stand for ever.
Jesus said that his words are eternal. (Matthew 24:34-35, Mark 13:31, Luke 21:33)
Matthew 24:34 Verily I say unto you, This generation shall not pass, till all these things be fulfilled. 35 Heaven and earth shall pass away, but my words shall not pass away.
Mark 13:31 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.
Peter confessed that the words of Jesus were the eternal words of God prophesied in the Old Testament. (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.