Not slow motion in specific, but the first example (or at least first notable example) of intercutting between slow motion and normal speed is in Kurosawa's Seven Samurai in the introductory scene of Kambei (Takashi Shimura) in which he deals with a troublesome and hungry thief. Here is the scene, about two minutes in (sidenote, notice how subtle it is, and how emotionally powerful--clearly more than just a visual gimmick). This was picked up by later action directors, particularly John Woo, who put his own deliriously indulgent spin on it. While the technique was by no means unused between them, it is in many ways from Woo (or more broadly, the Hong Kong action cinema he so exemplified) that a lot of modern directors take their cue.

This isn't slow motion in general of course, but one of the more common ways directors play around with film speed to create an effect. My source on this is Stephen Prince's The Warrior's Camera: the Cinema of Akira Kurosawa.

Although it's hard to parse out who was first, the Soviet filmmaker Sergei Eisenstein pioneered both slow motion and accelerated motion uses an aesthetic tool for filmmaking. Eisenstein would use both slow and accelerated motion to better convey a sense of temporality and the "moment" among the audience. The Battleship Potemkin's famous Odessa steps sequence uses a mixture of accelerated and decelerated time to convey the crowd's panic at the brutality of the tsarist soldiers. Some of this is ersatz slow motion; for example having some actors be much slower in their pantomimes when focusing on their faces or alternating between quick and slow panning shots of the crowds' flight. The result is to create a degree of disorientation that causes the viewer to identify with the crowd.

Although I doubt Eisenstein was the first to use these techniques, he was one of the first filmmakers to have an explicit artistic rationale for using accelerated and decelerated time to achieve an overall artistic effect.