Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[315] So the Hebrews went out of Egypt, while the Egyptians wept, and repented that they had treated them so hardly. - Now they took their journey by Letopolis, a place at that time deserted, but where Babylon was built afterwards, when Cambyses laid Egypt waste: but as they went away hastily, on the third day they came to a place called Beelzephon, on the Red Sea; and when they had no food out of the land, because it was a desert, they eat of loaves kneaded of flour, only warmed by a gentle heat; and this food they made use of for thirty days; for what they brought with them out of Egypt would not suffice them any longer time; and this only while they dispensed it to each person, to use so much only as would serve for necessity, but not for satiety. Whence it is that, in memory of the want we were then in, we keep a feast for eight days, which is called the feast of unleavened bread. Now the entire multitude of those that went out, including the women and children, was not easy to be numbered, but those that were of an age fit for war, were six hundred thousand.