Someone told me that if a creature has zero movement speed and you use a forced movement ability on them (like thunwavwave), they will be knocked prone.
It is not true. In fact, it's so not true, forced movement is popular as a solution to being grappled (the most common way in the game to have speed 0), because there are no downsides, including the fact that it won't knock the target prone.
0 movement speed just means they cannot move. If an ability says it pushes them in a direction and that is the only thing it says, then it pushes them. Spells and abilities do exactly what they say they do. Hitting someone with 0 movement speed does nothing special.
There might be some confusion here, as there are two types of forced movement - spells that move you (thunderwave) and spells that force you to use your own movement (dissonant whispers).
With spells that move you, it doesn't matter how much movement you have left because an outside force is moving you. With spells that cause you to use your movement, there's generally a workaround since the creature isn't moving on its own turn. For example, dissonant whispers says a creature who fails its save "must immediately use its reaction, if available, to move as far as its speed allows away from you."
In any case, there's no "0 movement = knocked prone" rule.
Someone told me that if a creature has zero movement speed and you use a forced movement ability on them (like thunwavwave), they will be knocked prone.
Is this true and where can I find it?
It is not true. In fact, it's so not true, forced movement is popular as a solution to being grappled (the most common way in the game to have speed 0), because there are no downsides, including the fact that it won't knock the target prone.
What about hitting someone with zero movement speed?
0 movement speed just means they cannot move. If an ability says it pushes them in a direction and that is the only thing it says, then it pushes them. Spells and abilities do exactly what they say they do. Hitting someone with 0 movement speed does nothing special.
There might be some confusion here, as there are two types of forced movement - spells that move you (thunderwave) and spells that force you to use your own movement (dissonant whispers).
With spells that move you, it doesn't matter how much movement you have left because an outside force is moving you. With spells that cause you to use your movement, there's generally a workaround since the creature isn't moving on its own turn. For example, dissonant whispers says a creature who fails its save "must immediately use its reaction, if available, to move as far as its speed allows away from you."
In any case, there's no "0 movement = knocked prone" rule.
What they said. 0 movement just means can't move.
If a flying creature has 0 movement, it falls to the ground prone unless it can hover. But that is it.