Rashi on Exodus 12:3/רש"י על שמות י״ב:ג׳

1דברו אל כל עדת SPEAK UNTO ALL THE CONGREGATION — But did Aaron speak? Has it not already been stated, (Exodus 7:2) “You (Moses) shall speak etc.”. But the explanation is: they paid respect one to the other, saying to each other, “Instruct me what to say”. and the divine communication in question issued from both of them, so that it was as though they were both speaking (Mekhilta).

2דברו אל כל עדת ישראל לאמר בעשר לחדש i. e. SPEAK to-day — on the first of the month — THAT ON THE TENTH OF THE MONTH they should take it (the lamb) (Mekhilta).

3הזה [ON THE TENTH OF] THIS [MONTH] — As regards the paschal lamb sacrificed in Egypt it had to be taken from the flock on the tenth, but this did not apply to the paschal-lamb offered by future generations (Pesachim 96a).

4שה לבית אבות A LAMB FOR THE HOUSE OF THEIR FATHERS — i. e. a lamb for one family (“family” here means the larger unit comprising the father or grandfather with his married sons and their families). Consequently, if they (the members of such a family) were numerous, one might think that they may take one lamb for all of them (irrespective of whether this would suffice to provide a piece as large as an olive, the minimum quantity, for each person)! Therefore Scripture states: שה לבית A LAMB FOR EACH HOUSEHOLD (Mekhilta).