Rabbi Eliezer said: Anyone who involves himself with Perek Shirah in this world, merits saying it in the World-to-Come, as it says, “Then Moshe will sing”; it does not say “sang,” but “will sing” in the World-to-Come.
And Rebbi said: Anyone who involves himself with Perek Shirah in this world — I testify that he is destined for the World-to-Come, and he is saved from the evil inclination, and from harsh judgment, and from the destroying Satan, and from all types of enemies, and from the birth pangs of Mashiaḥ, and from the judgment of Gehennom; and he merits to learn and to teach, to observe and to fulfill and to perform [the Torah], and his studies are established in him, and his days are lengthened, and he merits life in the World-to-Come.
[Yalḳut Shimoni, end of Psalms:] The Sages said concerning King David that when he completed the book of Psalms, he became proud. He said before the blessed Holy One, “Is there any creature you have created in your world that says more songs and praises than I?” At that moment a frog happened across his path, and it said to him: “David! Do not become proud, for I recite more songs and praises than you. Furthermore, every song I say contains three thousand parables, as it says, ‘And he spoke three thousand parables, and his songs were one thousand five hundred.' And furthermore, I am busy with a great mitsvah, and this is the mitsvah with which I am busy: there is a certain type of creature by the edge of the sea whose sustenance is entirely from [creatures living in] the water, and when it is hungry, it takes me and eats me, such that I fulfill that which it says, ‘If your enemy is hungry, feed him; if he is thirsty, give him water to drink; for you shall heap coals of fire on his head, and YHVH shall reward you'; do not read ‘shall reward you’ but instead ‘shall make him complete you.’”