You can listen to the recording on the article.
From the text:
The recording, which first aired on CNN’s “Anderson Cooper 360,” includes new details from the conversation that is a critical piece of evidence in special counsel Jack Smith’s indictment of Trump over the mishandling of classified information, including a moment when Trump seems to indicate he was holding a secret Pentagon document with plans to attack Iran.
It was provided by reporter(s) that were with Trump gathering background for Mark Meadows book. He knew they were there, who they were and presumably he was being recorded, or at least being transcribed, and felt it appropriate to share those files. Assuming they and the aides in the room are on the witness list, which apperently will be unsealed unless the Special Council appeals today’s ruling.