Exodus 12:21

And Mosheh called all the elders of Israel, and said to them, Withdraw your hands from the idols of the Mizraee, and take to you from the offspring of the flock, according to your houses, and kill the paschal lamb.

THEN MOSES CALLED FOR ALL THE ELDERS OF ISRAEL, AND SAID UNTO THEM: DRAW OUT, AND TAKE YOU LAMBS ACCORDING TO YOUR FAMILIES AND SLAUGHTER THE PASSOVER. This chapter shortens the account of how the laws which G-d had commanded Moses, as stated in the section above, [were communicated by him to Israel], as it is self-understood that Moses related all the laws to Israel in detail and taught them the matter involved, it being included in the verse, As the Eternal hath commanded Moses and Aaron, so did they. Instead, Scripture mentioned this section in a general way, saying that Moses called for all the elders of Israel and they gathered together to him all the people. Then they [the elders], etc.” said to the whole congregation of Israel, “Draw out the sheep from the flock to your homes, and keep it there from the tenth day of the month [till the fourteenth, when it is to be slaughtered as the Passover-offering].”
It is possible that Scripture used the word, mishchu (draw out), because their sheep were very far from them in the land of Goshen, for every shepherd is an abomination unto the Egyptians. It said, and take you, meaning “take the lambs according to your families,” every man a lamb, according to their fathers’ houses, and slaughter the Passover lamb at eventide, all in accordance with what has been explained above concerning this commandment.
Now we read in the Mechilta: “Then Moses called for all the elders of Israel. This teaches us that he constituted them a court. (the elders), “and zakein denotes only one who has acquired wisdom” (Kiddushin 32 b). See Ramban above, Verse 2, that “elders” are needed for the Sanctification of the New Moon, as mentioned in the section above, and hence Moses constituted them a court. And he said unto them. The word came from the mouth of Moses, saying it to all Israel., etc. These are the words of Rabbi Yashiya. Rabbi Yonathan says that the word came out from the mouth of Moses saying it to the elders, and the elders saying it to all Israel.” Thus according to Rabbi Yashiya, the expression, Then Moses called for all the elders of Israel, means that he told them to gather together to him all the people, [and he himself told the people all the laws of the Passover mentioned above], as I have explained. But according to the opinion of Rabbi Yonathan, the elders related it to the assembly. Accordingly, the verse stating, Speak ye unto all the congregation of Israel, refers [not to Moses and Aaron mentioned in the two verses there above, but] to the elders that were assembled before [Moses and Aaron, and they — the elders — spoke to the congregation], as is also the purport of the verse, Then it shall be, if it be done in error by ‘the eyes of the congregation,’ [which is a reference to the elders of the congregation, members of the Court].

למשפחותיכם, a variation of verse 3 where the wording had been לבית אבות.