Thus saith the LORD God of Israel… – various scriptures

If he be God’s minister, he does not found his teaching upon his own authority, for then his message would be only that of himself, and not to be esteemed; but he shows the authority of his Master, and none can gainsay him. He claims men’s attention on the ground that he utters a “Thus saith the Lord.” No matter how aged he may be, he does not proclaim the truth as merely the result of his long investigations or his extraordinary experience, but he grounds it upon “Thus saith the Lord.” So spake the hoary-headed Joshua when for many a year he had known the faithfulness of God and was about to die. He was singing his swan-song, preaching his last sermon; but he did not commence it, “Thus saith my age,” “Thus say I upon mine own authority,” but “Thus saith the Lord God of Israel.” A God-sent minister is the ambassador of the Most High, but he has no right to go beyond his commission; and when he does so, his office cannot yield him support. The prophets of God did not say, “Thus I speak as a prophet,” but, “Thus saith the Lord.” When the prophet came in Gideon’s days and spoke to erring Israel, he opened his mouth with, “Thus saith the Lord God of Israel.” Turn to the pages of Isaiah, and mark how frequent he quotes the divine authority; study the plaintive words of Jeremiah, and observe how solemnly his prophetic woes are prefaced with, “Thus saith the Lord;” and the soaring Ezekiel, to whom was given, as it were, six wings, that he might take more lofty flights than the eagle knoweth-even he relied not upon the sublimity of his language or the glory of his imagery, but found the sinews of his strength in “Thus saith the Lord God.” This is the trowel and this the hammer of God’s builders, -this the trumpet of his watchmen and the sword of his warriors. Woe to the man who comes in any other name! If we, or an angel from heaven, shall preach unto you anything but a “Thus saith the Lord,” no matter what our character or standing, give no heed to us, but cleave unto the truth as it is in Jesus. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0591.cfm