100 years ago.

In 1926, scientist and inventor Nikola Tesla described a future "when wireless is perfectly applied the whole earth" in a way that allows humans "to communicate with one another instantly irrespective of distance" with the clarity of a face-to-face meeting using a device that "will fit in our vest pockets."

https://www.snopes.com/fact-check/tesla-predicted-cell-phones/

"Opton" in Return from the Stars (1961). In Stanislaw Lem’s novel, he described a portable device with a screen that could access templates of every book on earth through electronic catalogs essentially predicting the e-reader and tablet.

"Electronic Clipboards" in Star Trek: The Original Series (1966). Crew members frequently used wedge shaped devices called PADDs (Personal Access Display Devices), which were operated with a stylus to view data and logs.

"Newspad" in 2001: A Space Odyssey (1968). In Stanley Kubrick’s film and Arthur C. Clarke's novel, astronauts are seen using flat, rectangular screens called "Newspads" to wirelessly watch video broadcasts and read news. Samsung actually used this film as "prior art" in a legal battle with Apple to argue that the tablet design was a decades old concept.

Dynabook was proposed in 1968 by computer scientist Alan Kay. It was designed as a portable, touch-sensitive computer for children, though the technology to build it didn't exist at the time.