Ball Bearings (bag of 1000)

As an action, you can spill these tiny metal balls from their pouch to cover a level, square area that is 10 feet on a side. A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone. A creature moving through the area at half speed doesn't need to make the save.

Source: System Reference Document 5.1 · CC-BY-4.0 · 5th Edition 2014