Exodus 16:4, 14

Then the LORD said to Moses, “Behold, I will cause bread to rain from heaven for you; the people shall go out and gather a day’s portion every day, so that I may test them [to determine] whether or not they will walk [obediently] in My instruction (law).

When the layer of dew evaporated, on the surface of the wilderness there was a fine, flake-like thing, as fine as frost on the ground.