Exodus 12:3, 5-6 (Darby Translation)
Speak unto all the assembly of Israel, saying, On the tenth of this month let them take themselves each a lamb, for a father's house, a lamb for a house... (5) Your lamb shall be without blemish, a yearling male; ye shall take it from the sheep, or from the goats. (6) And ye shall keep it until the fourteenth day of this month; and the whole congregation of the assembly of Israel shall kill it between the evenings.

THE WORKS OF PHILO

​The Decalogue XXX.159
...and the day called by the Hebrews in their own tongue the Pasch on which the whole people sacrifice, every member of them, without waiting for the priests, because the law has granted to the whole nation one special day in every year the right of the priesthood and of performing the sacrifices themselves.

Special Laws 2.XXVII.145-146
After the New Moon comes the fourth feast, called the Crossing-feast, which the Hebrews in their native tongue call Pascha. In this festival many myriads of victims from noon till eventide are offered by the whole people, old and young alike, raised for that particular day to the dignity of the priesthood. For at other times the priests according to the ordinances of the law carry out both the public sacrifices and those offered by private individuals. But on this occasion the whole nation performs the sacred rites and acts as priest with pure hands and complete immunity. The reason for this is as follows: the festival is a reminder and thank-offering for that great migration from Egypt which was made by more than two millions of men and women in obedience to the oracles vouchsafed to them. Now at that time they had left a land brimful of inhumanity which made a practice of expelling strangers, and what was worst of all, assigned divine honours to irrational creatures, not merely domesticated animals, but even wild beasts. So exceedingly joyful were they that in their vast enthusiasm and impatient eagerness, they naturally enough sacrificed without waiting for the priest. This practice which on that occasion was the result of a spontaneous and instinctive emotion, was sanctioned by the law once in every year to remind them of their duty of thanksgiving. These are the facts as discovered by the study of ancient history.

Life of Moses II.224-225
In this month, about the fourteenth day, when the disc of the moon is becoming full, is held the commemoration of the crossing, a public festival called in Hebrew Pasch, on which the victims are not brought to the altar by the laity and sacrificed by the priests, but, as commanded by the law, the whole nation acts as priest, each individual bringing what he offers on his own behalf and dealing with it with his own hands. Now while all the rest of the people were joyful and cheerful, each feeling that he had the honour of priesthood, there were others passing this time in tears and sorrow.

QA on Exodus, Book 1.10
Now at other times the daily priests (chosen) from the people, being appointed for the slaughtering and taking care of them, performed the sacrifices. But at the Passover, here spoken of, the whole people together is honoured with the priesthood, for all of them act for themselves in the performance of the sacrifice. For what reason? Because, in the first place, it was the beginning of this kind of sacrifice, the Levites not yet having been elected to the priesthood nor a temple set up. And in the second place, because the Saviour and Liberator, Who alone leads out all men to freedom, deemed them (all) equally worthy of sharing in the priesthood and in freedom as well, since all who were of the same nation had given evidence of equal piety.