Where does it come from (what is the makor) the minhag of saying Shiras ha-Yam in shul by chatzos on Shevi’i shel Pesach? Or some that have the minhag to say it before neitz early in the morning.
To my knowledge, the source for the communal recitation is from Yesod Veshoresh Ha-Avodah; the original source is the Arizal, who does not mention saying it communally, but only on one’s own.
The custom of saying the Shirah communally was adopted by some Chassidic communities, and among the old Jerusalem community in Sha’arei Chesed, where Rav Yaakov Moshe Charlap was the central focus of the affair.
It has become somewhat popular today even among regular “litvish” circles, for sociological reasons worthy of investigation.