The LORD instructed Moses that the people should gather only a day's portion of bread (manna) as a test of their obedience to His command. Exodus 16:4 (RSV)

4 Then the Lord said to 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.