Charles Ellicott Commentary Isaiah 26:4

"Trust ye in Jehovah for ever; for in Jehovah, [even] Jehovah, is an everlasting rock." — Isaiah 26:4 (ASV)

For in the Lord Jehovah. —The Hebrew presents, as in Isaiah 12:2, the exceptional combination of the two names Jah (Psalms 68:4) and Jehovah. In the Hebrew for everlasting strength, we have, literally, the Rock of Ages of the well-known hymn. We have the same name of Rock applied to express the unchangeableness of God, as in Deuteronomy 32:4.