The recently released affidavit is the source for this. He was told to return the docs, claimed he returned them all, and in fact when the search was conducted it was found that he didn’t.

Trump was given 18 months to return them with multiple requests, attested through a lawyer in an affidavit that he did, and yet did not. Then when caught he has made claims that they were his documents and that’s why they were not returned. So far he hasn’t made any defense that is was an accident or an oversight. This shows the possession of the docs was willful, and intentionally not returning them was obstruction.