| It's in the PHB Errata:
Prone [Addition]
Player’s Handbook, page 277
Add “You can’t move from your space, although you can teleport, crawl, or be forced to move by a pull, a push, or a slide.”
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"I am King of the Romans and above the rules of grammar!" - Sigismund, Roman Emperor and my new hero
"Craft and profession skills are a tax on people who believe characterization and back-story are important." - Obryn
"Another thing to keep in mind is that the typical D&D party is to medieval armies what the A-team is to modern police." - Eamon
"I realize falling isn't specifically a push, pull, or slide, but I'd heavily argue that gravity is forced movement." - Old Gumphrey | 
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