Oh, I think it's quite clear where he came up with that idea. Right before his speech, there was a presentation from the CDC about the effect of cleaning agents and sunlight on hard surfaces in terms of protecting against COVID-19. (The video is available on line.) He heard the presentation, and then when he got behind the podium, decided to say something that would indicate how much attention he was paying and what an expert on infectious disease he was.
Funny aside - at one point he asked Dr. Birx whether or not that was an effective treatment. She said "no." When the White House released the transcript of the speech, they had changed her answer to "yes."
