I know he said this prior to March 25, 1850 as quoted by Mr. Bay in the Democratic Banner, Bowling Green, Pike County, Missouri, but it is recorded again in a speech in the Senate of the United States, February 24, 1854, ...
"Sir, I have no fellowship with that sickly sentimentality that speaks of slavery as a great moral evil, and is constantly praying for some safe and peaceful mode of getting rid of it. I believe that slavery is of divine origin, and that it is a great moral, social, and political blessing —a blessing to the slave, and a blessing to the master. I am not going to elaborate this idea; it is of itself a theme for half a dozen speeches. But I undertake to say, that nowhere, in all Christendom, is there a higher degree of morality than in the slaveholding states. In this respect the slaveholding states challenge a comparison with their boastful sisters of freedom. I risk nothing in saying that slavery operates as a check upon crime. I will tell you why. It equalizes white men, puts them on a level with one another, and represses thereby many of the evil passions which rise up and drive men to madness in communities where white men are not equal."
"He" being Albert G. Brown, not Jefferson Davis.
Page 334, Speeches, Messages, and Other Writings of the Hon. Albert G. Brown, of Mississippi.
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Atlanta Confederacy, 1860: "We regard every man in our midst an enemy to the institutions of the South, who does not boldly declare that he believes African slavery to be a social, moral, and political blessing."
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Again the same like words are used by Mr. Butler of South Carolina in the Edgefield Advertiser of March 06, 1840.
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Again from the Southern Patriot, Houston, Chickasaw County, Mississippi of August 29, 1849...Owen Van Vacter at Oakland College, Mississippi, April last..."The institution of slavery, then- however reviled by ignorant and unprincipled - is a social and political blessing and rests, for it morality, on the highest authority.
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From the Madisonian, Washington City (D.C.) Dec. 18, 1843.
Annexation of Texas- Public Meeting.
From the Canton Miss. Democrat
Report of Resolutions of the people of Madison County, Miss. on the subject of the Annexation of Texas.
Resolution #7.
"That so far from considering domestic slavery an evil, calling for all the sickly sentimentality and bitter denunciation of Christendom - we regard it as a moral and political blessing, coeval with and necessary to the progress of society from a state of barbarism, clearly recognized by divine revelation, and an important element in the grand scheme of civilization."
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I assume if my newspaper search engine went further back there would be more.