I don’t think the show ever depicted him presenting it, but it was definitely mentioned in conversation that before joining the Stargate program, he had been ostracized from the archaeological community for his wild alien landing pad hypothesis. I cannot remember which episodes this was mentioned.

EDIT:

SG-1 S3E21

CARTER: The academic community didn’t believe your theory that the pyramids were landing sites for alien spaceships either.

SG-1 S4E14

STEVEN: As I recall, the last time I saw you, you were giving a lecture to an empty room. DANIEL: Well, it was full when I started. STEVEN: Well, maybe the world wasn’t ready to hear that the pyramids were built by aliens…

SG-1 S8E8

COLSON: Oh Doc! I’m a big fan. DANIEL: Really? COLSON: Yes, I’ve been following you since your early career. Pyramids as landing pads for alien ships? Fascinating.

SG-1 S8E19

HAMMOND: You have a background in archaeology, isn’t that right, Doctor Jackson? DANIEL: That’s right. HAMMOND: But your theories failed to gain much acceptance among your colleagues. DANIEL: You could say that. HAMMOND: Egyptian pyramids as landing pads for alien spacecraft.

Continuum

LANDRY: And you, Doctor Jackson, were last seen in Egypt, attempting to find proof that aliens once visited Earth and that the pyramids are landing pads for their spaceships… I take it that in your timeline that you’re not a discredited whack-job living on the fringes of society.