John Gill Commentary Psalms 89:2

"For I have said, Mercy shall be built up for ever; Thy faithfulness wilt thou establish in the very heavens." — Psalms 89:2 (ASV)

For I have said That is, in his heart he had said, he had thought of it, was assured of it, strongly concluded it, from the Spirit and word of God; he believed it, and therefore he spoke it; having it from the Lord, it was all one as if he had spoke it: