Disregarding a journalistic sine qua non -- Establish the source's credibility -- The Post succeeded recently in elevating what might have been a routine story, if it was one at all, beyond its real news value. Then, with a well-intentioned effort the next day to correct the lapse, the newspaper compounded it.

This begins with an Oct. 13 report in which one Franklin Lamb, "an international law specialist," charged that Israel had "flagrantly violated" U.S. law by "extensive use" of cluster bombs against civilians in Lebanon. With his evidence, the story said, Mr. Lamb "hopes to persuade" the Reagan administration to fix permanently its temporary prohibition against shipment of these controversial armaments to Israel.

Some background:

Mr. Lamb contacted The Post's foreign desk a few weeks ago, saying he wanted to submit his findings on a month's research in Lebanon. He was referred to George Wilson, longtime correspondent for the national desk and knowledgeable about munitions. Mr. Wilson met twice with Mr. Lamb, whose evidence included affidavits from doctors who treated civilians for cluster-bomb injuries, interviews with victims, spent bomb parts and posters warning Beirut residents against unexploded elements.

Before writing his story, Mr. Wilson showed a photograph of a poster to the Israeli military attache in Washington. The attache confirmed use of the bombs pictured, insisting they were used against military targets only. Other evidence was checked with Defense Department specialists.

Remember that eyewitness reports that cluster bombs had damaged non- military buildings and caused civilian casualties have circulated in the media, including The Post, since mid-June. Since then, the Israeli government has acknowledged their use, but steadfastly denies that civilians were targeted. Consistent with its restriction on more deliveries, the administration will say only that the issue "remains under review."

The initial article described Mr. Lamb also as "a former college lecturer active in civil rights causes in the 1960s" and said that his trip to Lebanon was paid for by the Arab Information Office, "which had urged him to investigate conditions" there. He is quoted as saying that no Arab groups were involved in preparation of a report.

Criticism was immediate and pointed. The Jewish Community Council of Greater Washington charged that The Post had been "duped." Most cited a Sept. 6 New Republic article that enumerated several published reports of alleged earlier wrongdoings and misrepresentations by Mr. Lamb. One involved a putative "congressional delegation," including Mr. Lamb, and rumors that Israel had used a "U.S.-built vacuum bomb." This report has been discredited. "No such weapon exists," the State Department has said.

The follow-up story Oct. 14, not written by Mr. Wilson, said The Post "was not aware of the controversies in Lamb's background." The Post reported several of The New Republic's charges, leaving the issue, as one critic observed, "a debate between Lamb and the magazine . .. absolving The Post." Inexplicably, it attributed a report that Mr. Lamb had once advocated establishment of "an independent Palestinian state" to the magazine, which in turn attributed it to The New York Times. Had The Post looked in its own files for March 31, 1980 and after, this and more was there. Thus, twice around, the paper ignored its own independent information.

Clearly, Mr. Lamb's actions inspire debate, and readers were entitled to know that. Whether the sweeping judgments of The New Republic or those in a Jewish Community Council letter to Executive Editor Ben Bradlee -- e.g. "'scholar' Lamb has a lengthy criminal record" -- would stand up is questionable. And shouldn't Mr. Lamb have an opportunity to explain himself?

Here, the fault rests with editors. The initial story was "in type" for a few days before publication. The assistant managing editor for national news, Peter Silberman, agrees it was a "crucial exception" not to have examined Mr. Lamb's bona fides and "it should have been caught at the desk." Mr. Wilson assumed Mr. Lamb had already "passed muster," and he was being asked to concentrate on the evidence. "I was not writing a story about Mr. Lamb," he says. He concludes that evidence "advanced the public record on cluster bomb use." Maybe so; but that has now been overshadowed by controversy about the source.