God spoke to Moshe and to Aharon in the land of Egypt, saying: let this new-moon be for you the first of the new-moons, the beginning-one of the months of the year. Speak to the entire community of Israel, saying: on the tenth day after this new-moon they are to take them, each man, a lamb, according to their ancestral house, one lamb per household. Now if there be too few in the house for a lamb, he is to take it, he and his neighbor who is near his house, by the accounting of the number of persons; each man according to what he can eat you are to slaughter a lamb. A wholly-sound male, year-old lamb shall be yours, from the sheep and from the goats are you to take it. It shall be for you in safe-keeping, until the fourteenth day of the same moon, and they are to slay it--the entire community of Israel--at the setting of the sun.
(ו) וָאֶעֱבֹ֤ר עָלַ֙יִךְ֙ וָֽאֶרְאֵ֔ךְ מִתְבּוֹסֶ֖סֶת בְּדָמָ֑יִךְ וָאֹ֤מַר לָךְ֙ בְּדָמַ֣יִךְ חֲיִ֔י וָאֹ֥מַר לָ֖ךְ בְּדָמַ֥יִךְ חֲיִֽי׃ (ז) רְבָבָ֗ה כְּצֶ֤מַח הַשָּׂדֶה֙ נְתַתִּ֔יךְ וַתִּרְבִּי֙ וַֽתִּגְדְּלִ֔י וַתָּבֹ֖אִי בַּעֲדִ֣י עֲדָיִ֑ים שָׁדַ֤יִם נָכֹ֙נוּ֙ וּשְׂעָרֵ֣ךְ צִמֵּ֔חַ וְאַ֖תְּ עֵרֹ֥ם וְעֶרְיָֽה׃ (ח) וָאֶעֱבֹ֨ר עָלַ֜יִךְ וָאֶרְאֵ֗ךְ וְהִנֵּ֤ה עִתֵּךְ֙ עֵ֣ת דֹּדִ֔ים וָאֶפְרֹ֤שׂ כְּנָפִי֙ עָלַ֔יִךְ וָאֲכַסֶּ֖ה עֶרְוָתֵ֑ךְ וָאֶשָּׁ֣בַֽע לָ֠ךְ וָאָב֨וֹא בִבְרִ֜ית אֹתָ֗ךְ נְאֻ֛ם אֲדֹנָ֥י וַתִּ֥הְיִי לִֽי׃
When I passed by you and saw you wallowing in your blood, I said to you, "By your blood, live; yes, by your blood, live." I let you grow like the plants of the field; and you continued to grow up until you attained to womanhood, until your breasts developed and your hair sprouted. You were still naked and bare when I passed by you again and saw that your time for love had arrived. So I spread My robe over you and covered your nakedness, and I entered into a covenant with you by oath and so you became Mine--declares the Holy One.
A "It shall be for you in safe-keeping, until the fourteenth day of the same moon" (Ex. 12:6).
Why did Torah require the purchase of the Pesach lamb four days before its slaughter?
B Rabbi Matia answers: Our prophet teaches: "I passed by you again and saw that your time for love had arrived" (Ezekiel 16:8).
This means, the time has arrived for God to fulfill the oath sworn to Avraham to redeem his children. But, the Israelites had not yet performed any mitzvot by which to merit the redemption, as the prophet also said: "You were still naked and bare" which means bare of mitzvot.
The Holy One gave them 2 mitzvot--the blood of Pesach and the blood of brit milah, so that the Israelites would do them and be redeemed, as the prophet said: "When I passed by you and saw you wallowing in your blood, I said to you, "By your blood, live; yes, by your blood, live" (Ez. 16:6).
The same idea is repeated by another prophet: "Also for you, due to the blood of your covenant (brit), I sent you out of your imprisonment, out from the pit which had no water in it" (Zecharia 9:11).
This is why the Israelites were told to set aside the Pesach lamb four days prior to its slaughter, because you only receives rewards based your deeds.