The phrase was discovered during the NSFW episode titled "But Cuba". The phrase was such a hit that there was no question that it would be the title of the episode.
Leo had heard the phrase, but did not know the origin. He was fully briefed about the origin of "but Cuba" during The Trials of King Leo and has since begun using the phrase himself.
But Cuba, the original episode of NSFW in which it appeared.
Tom Merritt slipped a "but Cuba" into his final episode of Buzz Out Loud.
The Trials of King Leo, in which the origin of the phrase is explained to Leo Laporte.
Numerous other NSFW episodes after the "But Cuba" episode.