The quote comes from the seventh episode of the first season of SpongeBob SquarePants, where the protagonist of the series becomes a Hall Monitor and wreaks havoc in Bikini Bottom.
One of the scenes present two police officers approaching Patrick Star, showing him a sketch of the Maniac, to which Patrick reacts with a shriek, every time he sees it.
One of the policemen then says “calm down son, it’s just a drawing” then proceeds to troll Patrick with the sketch.
The YouTube channel Nickelodeon UK uploaded the clip on December 28th, 2016.
Image macros of “calm down son, it’s just a drawing” would appear on Reddit, in 2018, spreading on to sites like 9GAG in later years.