I thought that was a zen buddhist adage.

It sure sounds like one, but I'm not sure if I'm remembering it accurately, or if there's anymore specific source than just "Zen Buddhists".

It’s a version of a Zen koan. The original is something like ‘ Before one studies Zen, mountains are mountains and waters are waters; after a first glimpse into the truth of Zen, mountains are no longer mountains and waters are no longer waters; after enlightenment, mountains are once again mountains and waters once again waters.” That is attributed to Dogen, who is the founder of the Sōtō Zen school.

The gist of the meaning is that the Zen student starts off a naive realist - ‘mountains really are mountains’. Then on learning the truth of śūnyatā (emptiness, dependent origination) the student discovers that mountains have no intrinsic reality, that they are ‘appearances only’. But, very much in the spirit of Sōtō Zen, in particular, having discovered this, one abandons all expectation of them being something other than what they simply are, so in the end they’re mountains again, although implicitly one’s understanding has been refined by the experience of satori such that one sees them ‘as they truly are’.

The pop singer Donovan wrote a song about this koan, ‘first there is a mountain’. The San Francisco Zen Centre, founded by Shunryu Suzuki in 1969, is a Sōtō Zen centre and has been central the dissemination of Dogen’s teachings through the wider culture. Dogen’s main philosophical book, Shobogenzo, is widely regarded as a classic of Japanese classical literature.

Thanks so much! Should have guessed you would be the one to ask about this.