Philo Judaeus De Decalogo 16

[16] but even of wholesome water, in order that, when after having found themselves in want of necessary food, and expecting to be destroyed by hunger and thirst, they should on a sudden find themselves amid abundance of all necessary things, spontaneously springing up around them; the heaven itself raining down upon them food called manna, and as a seasoning delicacy to that meat an abundance of quails from the air; and the bitter water being sweetened so as to become drinkable, and the precipitous rock pouring forth springs of sweet water; then they might no longer look back upon the Nile with wonder, nor be in doubt as to whether those laws were the laws of God, having received a most manifest proof of the fact from the supplies by which they now found their scarcity relieved beyond all their previous expectations;