Which literary device does this contain and set a thousand leafy tongues a-whispering all at once?

The literary device used in the phrase "set a thousand leafy tongues a-whispering all at once" is personification.

Here's why:

* Personification: Giving human qualities or characteristics to inanimate objects or abstract concepts.

In this case, the leaves of trees are given the human quality of having "tongues" and "whispering." This creates a vivid and imaginative image of the rustling sound of leaves in the wind.