PHB, page 151. Available in the SRD.
Adventuring Gear
Varies
As an action, you can spill these tiny metal balls from their pouch to cover a level area 10 feet square. 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 saving throw.
Cost: 1 gp
Weight: 2 lb