Trump said that in May 2026 , when he was talking about the Reflecting Pool’s new lining and said that “if you had a knife, you can’t even cut it.” A later report notes that he made that claim about 48 days before he shifted to blaming vandals for the damage.

What was said

He described the liner as very strong and said a knife could not cut it, comparing it to “powerful rubber” and saying it was “sealed”.

Why people noticed it

The line drew attention because later reports said the pool liner had been cut with a sharp knife or razor, which made the earlier claim look contradictory.

Bottom line

So the answer is: he said it in May 2026 , during remarks about the Reflecting Pool renovation, well before the June 2026 reporting on the alleged damage.