According to Next Draft
Trump says he and Putin discussed outcome of Mueller probe as part of hour-long phone conversation. "I sort of smiled when he said something to the effect that it started off as a mountain and ended up as a mouse." (Psst. Putin hacked our elections...)
Some Latin scurried across my memory—mons partitur ridiculus mus nascitur. Remembering an earlier misadventure, I decided to check. What Horace really wrote was parturiunt montes, nascetur ridiculus mus. It's in Ars Poetica.