What do we learn from the verse?
Rashi: Even fetuses 1 sang Shirah at Keri'as Yam Suf. Sotah 31a - the womb became like a glass via which they were able to see the miracle.
Brachos 49b: The text of the Zimun (introduction to Birkas ha'Mazon) varies according to the number of people.
Kesuvos 7b: We need 10 to bless Birkas Chasanim.
Radak: We should bless Hashem amidst multitudes, to publicize His wonders to all.
Malbim: With them went many congregations who blessed Elokim and Hashem 2 .
Why does it say "mi'Mekor Yisrael"?
Radak: This is everyone who came out of Mekor [the womb of] Yisrael. This is like "Kol Zera Yakov Kabduhu" (22:24).
Malbim: It is in Eretz Yisrael. It compares the camps that went to Chutz la'Aretz to fight, which were also congregations, to water that flowed outside its source. We who go to greet them from Eretz Yisrael, are depicted as the source itself full of great water, and congregations without number.
