Charles Ellicott Commentary Exodus 12:17

"And ye shall observe the [feast of] unleavened bread; for in this selfsame day have I brought your hosts out of the land of Egypt: therefore shall ye observe this day throughout your generations by an ordinance for ever." — Exodus 12:17 (ASV)

In this selfsame day have I brought your armies out of the land of Egypt. —On the application of the word “ a rmies” to the people of Israel, see above (Exodus 6:26). The expression “have I brought” indicates either that these directions were not given until after the Exodus, or at any rate that they were not reduced to writing until then.