“…And Aharon stretched forth his hand over the waters of Egypt, and the frog came up and covered the land of Egypt…..” – Shemot 8:2

וַיֵּט אַהֲרֹן אֶת יָדוֹ עַל מֵימֵי מִצְרָיִם וַתַּעַל הַצּפַרְדֵּעַ וַתְּכַס אֶת אֶרֶץ מִצְרָיִם – שמות ח, ב

Our Sages place great emphasis on the importance of executing a mitzvah project from start to finish and not leaving it incomplete (see Rashi on Devarim 8:1).

However, when in doubt what constitutes the mitzvah’s complete fulfillment, the appropriate course of action is not always uniform. In instances where excessiveness is not a bad thing, such as when giving charity, extending your involvement in the project is obviously ideal, in order to avoid the possibility of leaving it incomplete.

There are instances, however, where excessiveness is wrong. For example, when Beit Din must inflict Malkot, lashes, upon transgressors of certain sins, the Torah warns, “He shall flog him with forty lashes; he shall not exceed, lest he give him a much more severe flogging than these (Devarim 25:3).” In such an instance, if we are unsure as to what constitutes the mitzvah’s completion, we suffice with fulfilling our duty by the minimum standards – since exacting excessive punishment is prohibited. In fact, the maximum amount of lashes inflicted for one sin is thirty-nine and not forty (as the verse would imply,) which according to some explanations is a rabbinic measure taken to avoid too close an encounter with exceeding the limit.

This explains the view of the Midrash regarding the manner in which the Plague of Frogs was set in to motion. Though Aharon had been commanded by G-d to inflict the Egyptians with frogs that will swarm their entire land, yet the Torah implies that Aharon actually brought up only one frog from the water: “Aharon stretched forth his hand… and the frog came up and covered the land.” The Midrash explains that indeed Aharon’s actions produced only one frog, but swarms more streamed from that frog when it was hit.

Since Aharon’s command involved inflicting pain on another human being, the Midrash understands that Aharon preferred to complete this mitzvah with the minimum possible involvement on his part. He therefore “sufficed” with bringing up only one miraculous frog and let the full brunt of the punishment be brought about by the Egyptians themselves.

—Likkutei Sichos vol. 16, pp. 84-85