This was a big moment from Sunday night's airing of "Once Upon a Studio" ... a short film the Mouse House released on ABC to celebrate their 100-year anniversary, to touch on all the iconic characters and milestones they've hit over the years -- RW's Genie, included.

The Genie's cameo is brief but memorable ... he's shown popping off a page that Olaf -- from "Frozen" -- is working on, which causes the snowman to go crashing to the ground.

In response, Genie says ... "I haven't seen a fall like that since Rome." Once Olaf pops up and recomposes himself, Robin can be heard saying ... "Much better." It's just a couple lines, but some fans worried Disney recreated the late actor's voice by using artificial intelligence.

Fortunately for everybody, that's not the case ... as clarified by the producers/directors -- who told Variety they got official permission to use the real thing ... namely, Robin's actual vocals.

They say they reached out to Robin's estate, explaining ... "We tried to take them on the journey with us to say, 'We’ve got this very special short that we’re doing. Robin as the genie means so much to so many people and we would really love to involve him.'"

The creative team behind 'OUAS' adds ... "So Dan listened to the outtakes from the original recording and he found those little bites that we could use. We went back to the estate and said, 'This is what we hope to do. Eric, who originally animated the genie is on the show, and he’s going to be part of it.' And, it was wonderful to see that happen."