Albert Barnes Commentary Exodus 16:4

"Then said Jehovah unto Moses, Behold, I will rain bread from heaven for you; and the people shall go out and gather a day`s portion every day, that I may prove them, whether they will walk in my law, or not." — Exodus 16:4 (ASV)

That I may prove them - The trial consisted in the restriction to the supply of their daily wants.