Last Updated: 7/3/2018
This easter egg can be seen in the 6th episode of season 1, titled, "Test Time".
During the scene where Koro-sensei begins tutoring the class using his super speed in order to get them ready for mid-terms, Ryoma notices and mentions the fact that Koro-sensei switches headbands for each subject he’s tutoring.
This is immediately followed by Koro-sensei appearing in front of him wearing not only a Naruto headband, but whiskers too, similar to those on Naruto’s face, which Ryoma recognizes and gets agitated over. He then exclaims, “Hey! Why do I get a Naruto headband?!” (subbed) or “Give me a break man, Naruto’s not even on the test!” (dubbed), showing that hes clearly aware of who Naruto is in their universe.
This same exact easter egg appears in Chapter 14 of the manga, also titled “Test Time”.