The line comes from The Empire Strikes Back, released in 1980, during Darth Vader's lightsaber duel with Luke Skywalker on Cloud City. Vader says it after Luke holds his own in the fight, recognising the teaching of Obi-Wan Kenobi, played in the earlier films by Alec Guinness. Voiced by James Earl Jones, the delivery became a favourite among Star Wars fans and later spread through meme edits, reaction videos and soundboards, where it is now used far beyond the original scene to praise anyone who has clearly learned from a great teacher, much like fellow Star Wars favourite Chewbacca.