Speak to the entire congregation of Israel, saying, 'On the tenth of this month each man shall take a lamb for a father's house, a lamb for a household...
The Ohr Hachaim asks why the Torah tells us to speak to the entire congregation; the possuk says already that Hashem spoke to them לֵאמֹּר – to say to others. There was no reason to tell them again to tell this to Klal Yisroel. Even if there was a reason to tell them again, why did the possuk not say this before the previous possuk, that this month was the first of the months?
Another question that the Ohr Hachaim points out is that Klal Yisroel are to be told לֵאמֹּר. To whom was Klal Yisroel supposed to tell this? They are the only ones who are commanded in this.
The Ohr Hachaim explains that there were two reasons to buy this sheep. The first reason is a positive one. Klal Yisroel uplifted themselves and showed how they were winning against their enemies. Their enemies saw how they were fighting their idol and winning. The second reason to buy a sheep was because Hashem had commanded them to do so, and they had to listen in order that they not be punished.
This possuk is referring to the second reason to buy a sheep. In this possuk, Klal Yisroel is commanded to buy a sheep, and the word used is דַבְרוּ – speak, which denotes a strong form of speech. They are told to be sure to buy a sheep, because Hashem said so.
They were also told to buy the sheep לֵאמֹּר – using a weaker expression. They were told to buy the sheep for a positive reason, that they would gain from it, and it would be for their own benefit. Their own spiritual level would be advanced, and they would become better people.
The Mitzvah of counting the months from Nisan and announcing the new month does not have a punishment attached, and is not addressed with the term דַּבְּרוּ. This is why the possuk only says דַּבְּרוּ after that Mitzvah. The Mitzvah of sanctifying the new moon is only said with a לֵאמֹר, because they get reward for keeping the Mitzvah without being punished for not keeping it.
