Based on release dates — rather than the in-universe timeline — when was the first time that one of the MCU movies referenced an event that took place in another part of the MCU other than a movie?
For example, an MCU movie referencing something that happened in one of the spinoff TV shows, or a tie-in comic.
Such as one of the movies referencing an event that happened in Agents of S.H.I.E.L.D., She-Hulk: Attorney at Law, or Loki.
I'm thinking of something that's explicitly part of the MCU canon as described in Marvel's MCU archive, as opposed to material that originates from Marvel but is outside of the MCU. So, not films like the X-Men or Logan or the 1980s/90s Thor or Hulk movies. Or the Marvel Anime series.