Translation of the source text.Pesikta Rabbati, ed. Meir Friedmann (Ish-Shalom), Vienna 1880 · Public Domain
"For the leader, upon the Gittith, of Asaph. Sing aloud to God our strength; shout to the God of Jacob" (Psalms 81:1-2). Let our master teach us: a shofar that was pierced and stopped up, is it fit to blow with? Thus our masters taught: if the repair impedes the blast, it is unfit; if not, it is fit. What is the reason? For it is written, "And the LORD God will blow the shofar" (Zechariah 9:14): just as the shofar of the Holy One, blessed be He, is whole, so the shofar of Rosh Hashanah must be whole. Why? Because on Rosh Hashanah they are redeemed from the Angel of Death. Rabbi Akiva said: the horn of a blemished animal is fit for blowing on Rosh Hashanah and on the Jubilee, as it is said, "Then you shall make the shofar blast pass through... throughout all your land" (Leviticus 25:9), but for sacrifices, no. And why do Israel blow the shofar on Rosh Hashanah? So that the Holy One, blessed be He, may judge them with mercy and find them meritorious, as it says, "God has ascended amid the blast, the LORD amid the sound of the shofar" (Psalms 47:6). Asaph said: Since by this shofar the Holy One, blessed be He, has mercy on Israel and finds [them] meritorious in judgment, be careful to take shofars on Rosh Hashanah and sound them before the Holy One, blessed be He, and He will have mercy through them and find (him) [you] meritorious in judgment. From where? From what we read in the matter: the psalm, "Sing aloud to God our strength; shout to the God of Jacob."
[Sing aloud to God, etc.] Rabbi Tanchuma bar Rabbi Abba opened thus: "He has not beheld iniquity in Jacob" (Numbers 23:21). What did Balaam see, that he mentioned Jacob and not Abraham or Isaac? Rather, he saw that from Abraham there arose the dross of Ishmael and the children of Keturah, and from Isaac the dross of Esau and his chiefs, but Jacob was wholly holy, as it is written, "All these are the tribes of Israel, twelve" (Genesis 49:28). Therefore he mentioned none of the patriarchs except Jacob: "Shout to the God of Jacob, selah."
Another interpretation: Why Jacob, more than all the patriarchs? Thus our masters taught: by the measure a person measures, by that measure it is measured to him. A parable of a king who had three friends and wished to build a palace. He brought the first and said: See this place where I wish to make my palace. That friend said to him: I remember this place as a mountain at first. He brought the second and said the same. He said: I remember it as a field at first. The king set him aside. He brought his third friend, and he said: I remember it as a palace at first. He said to him: By your life, I will build that palace and call it by your name. So Abraham, Isaac, and Jacob were the friends of the Holy One, blessed be He. Abraham called the Temple a mountain, as it says, "On the mountain of the LORD it shall be seen" (Genesis 22:14). Isaac called it a field, as it says, "See, the smell of my son is like the smell of a field" (Genesis 27:27). And Jacob called it a palace, as it says, "How awesome is this place; this is none other than the house of God" (Genesis 28:17). The Holy One, blessed be He, said to him: You called it a house before it was built; by your life, I will build it and call it the house of Jacob: "O house of Jacob, come and let us walk in the light of the LORD" (Isaiah 2:5). And likewise, "And many peoples shall go... to the house of the God of Jacob" (Isaiah 2:3). And Jeremiah too says so: "Behold, I will restore the fortunes of the tents of Jacob" (Jeremiah 30:18). Asaph too set the words straight and mentioned only Jacob in the blast, as it is said, "Shout to the God of Jacob."
"Blow the shofar in the month, at the covered time, for the day of our festival" (Psalms 81:3). What is "at the covered time" [ba-keseh]? In the month that is covered. Another interpretation: on the New Moon that has the Festival of Sukkot in it. Another interpretation: What is "Blow the shofar in the month" [ba-chodesh]? Rabbi Berekhiah the priest said: When you take the shofar to blow, renew [chaddeshu] your deeds and repent; and even if you have many transgressions in your hand, I cover them over, as it is said, "Who is a God like You, bearing iniquity and passing over transgression" (Micah 7:18), "He will again have compassion on us; He will subdue our iniquities" (Micah 7:19).