Exodus 16:12

Hearing I have heard the murmurings of the sons of Israel before Me. Speak thou with them, saying, Between the evenings (suns) you shall eat flesh, and in the morning shall you eat bread, and shall know that I am the Lord your God.

I HAVE HEARD THE MURMURINGS OF THE CHILDREN OF ISRAEL. SPEAK UNTO THEM, SAYING: AT DUSK YE SHALL EAT FLESH. This communication [at dusk ye shall eat, etc.], had already been conveyed by Moses to Israel, but it is repeated here only because of that which He said I have heard the murmurings of the children of Israel. At first, He had said, Behold, I will cause to rain bread from heaven to you;, Verse 4. He would willingly do it for them as an act of kindness or because of their merits. But now He said that [their murmuring] is accounted to them as sin, and yet because of the very nature of the murmuring, He will do such things for them so that they might know thereby that I am the Eternal your G-d. Until now ye do not believe the Eternal your G-d; this is why you murmur against His prophets.
It is possible that at first He did not promise them the manna for all the time that they would be in the wilderness. Thus they thought that perhaps it would be for one day or two, as long as they stayed in that place, and that when they would leave it, they would come to a place of food. But now He said to them, “At dusk ye shall eat flesh always, and in every morning ye shall be filled with bread as long as you will be in the wilderness.” And so also is the opinion of our Rabbis, that the quail were with them from that day on, just like the manna. It is logically so, for they expressed discontent about the two things — [flesh and bread]. — and He hearkened to their murmurings on both matters, and He gave them that which they craved. What would He give them, and what more,, 120:3. if He supplied them with meat for only one day or two?
The reason that the chapter [of the Torah] speaks at length about the matter of the manna is that everything about it was of a wondrous nature, whereas with reference to the matter of the quail, it just writes briefly, And it came to pass at even, that the quail came up, because it came in a natural way. [Although the quail were a daily occurrence], the subject of the second [incident of] quail at Kibroth-hata’avah [is singled out for mention in Scripture] because right now [in the wilderness of Sin], they did not receive of them to the full, just as He always says here, flesh to eat, and bread to the full.. This clearly indicates that bread they had to the full, but not flesh.
It is possible that only the adults gathered the quail — or it may be that they were marked by chance for the pious ones — while the young craved and hungered for them, since Scripture does not relate concerning the quail, “and they gathered some more, some less,” as it does concerning the manna. It is for this reason that Scripture says there [in the narrative of the second quail]: And the mixed multitude that was among them fell a lusting, and it further says, and the children of Israel also wept on their part, meaning that some of the children of Israel were also weeping for it, but not all. He then gave them in great quantity, as it says, He that gathered least gathered ten heaps,, Verse 32. and out of that abundance, they ate for a month’s time, Verse 20. and then the quail reverted to their first state.
In line with the plain meaning of Scripture, the whole affair with the quail happened only at intervals, but since the manna was their staple food, they always had it, for their chief murmuring concerned it, as it is written, for ye have brought us forth into this wilderness, to put to death this whole assembly by famine.

וידעתם כי אני ה' אלוקיכם, Who has taken you out of Egypt.