The typical pear-shaped Grey, if it doesn't originate with it, was at least popularized by the Whitley Streiber book Communion, which purported to be true account of abduction. The cover art is basically the popular idea now.

EDIT: u/SleepPellest has brought up Close Encounters of the Third Kind, whose depiction precedes Communion by ten tears and likely influenced it. I had forgotten all about that. There are also several different alien designs in the original Star Wars cantina scene that feature big heads and black or dark bulbous eyes. You can see some similarities between these guys and both Spielberg's tall alien and the popular depictions.

As far as green aliens and the "little green men" idea goes, that goes back to the 1950s or earlier. I'm sure others out there know better than I, but aliens being green comes up in lots of depictions from Burrough's John Carter series to comic book characters (Martian Manhunter, Skrulls) to things like Star Trek with its green slave girl and even Spock who has green blood which gives his skin a different tone (in the earliest conceptions, Spock was thought to be Martian). Some of this may aldo come from other abduction accounts, but I don't know much more about it.

My first thought was Communion, but then I remembered Close Encounters having preceded the book by about six years. The iconic alien on the cover seems to have been---dunno if this is quite the right word, because my intention isn't to call bullshit on Strieber---influenced by both the short and tall creatures at the end of Spielberg's movie.

Don't know of any depiction of the standard "grey" before the film.

The "ancient text" in question is the Betty and Barney Hill Abduction case - it's where a lot of alien lore has its source, including their physical appearance and the idea that they come from Zeta Reticuli.

Their case was popularized/legitimized by physicist Stanton Friedman who, along with Dr. Allen Hynek, was one of the consultants for the film Close Encounters of the Third Kind. Friedman's book on the case, Captured: The Betty and Barney Hill UFO Experience has all kinds of stuff in it that'll seem familiar to folks who grew up on The X-Files and Close Encounters.

That's not the first influential book on the Hill case (that's The Interrupted Journey, by John Fuller), but it ties together a lot of elements, including some that came out years after the incident.

That's kind of in the nature of the case - the Hills only remembered/reconstructed/described quite a few things years after they happened after several hypnotic sessions.

If you're unfamiliar, the rough outline of the Hill abduction goes:

In 1961, interracial couple (this becomes an issue in some skeptical analyses) and government employees Betty and Barney Hill are driving home to New Hampshire from a trip into Canada when they notice a strange light doing maneuvers in the distance.

After some time, it draws nearer to their car - Barney pulls over and looks at it through his binoculars, and can see that it's not an ordinary plane and that there are some kind of occupants.

He gets back in the car and starts driving. Then, they realize they can't see the light any more. And two hours have passed. And they are 35 miles further down the road. No idea where the time has gone or how they've gotten there.

The next day, they report the incident to the nearby Air Force Base. Over subsequent weeks, Betty begins having nightmares about something that happened that night - she isn't sure exactly what, but it's disturbing.

Major James MacDonald suggests they undergo hypnotic regression to try to figure out what happened, and the story that gradually emerges over several sessions is what's now recognizable as the archetypal alien abduction: creatures that were human-like somehow paralyzed the two of them, brought them (perhaps by levitation) into some kind of a dark chamber aboard the craft, examined them coldly with some kind of medical equipment, showed them a few things out the windows and in the control room (like star charts where they were from), then returned them to their car.

Barney Hill made sketches of the flying disc-shaped craft; Betty sketched the star charts (which were later found to match the Zeta Reticuli system, or so the story goes), and a sketch artist drew the large-eyed, nearly noseless aliens during one of the regression sessions. Barney spoke about the beings' eyes staring at him - the idea of being watched and examined was a big part of his experience, at least subjectively: ""I felt like the eyes had pushed into my eyes."

The Hills also described the beings as more human-like before the regression (they remembered seeing uniformed figures), but during hypnosis, they described the figures as more like the ones we know and love today.

Skeptics, summarized fairly enough on Wikipedia, have pointed out that the aliens and craft seem kind of similar to some depictions in the media, especially from an episode of the TV show The Outer Limits that aired before the key regression session.

There's also a theory - I think Whitley Streiber mentions this one, but I can't recall who originated it - that the typical grey alien face is an exaggerated human face, perhaps as perceived under the influence of psychedelic drugs or during a neurologically triggered episode (Dr. Rick Strassman has pointed out potential links between alien abduction experiences and the experiences clinical subjects report when dosed with DMT).

EDIT TO ADD: I've just linked to the UNH archive of the case above; here's a painting of the alien as described by Betty under hypnosis, and a pencil sketch.