John Calvin Commentary Exodus 15:1

"Then sang Moses and the children of Israel this song unto Jehovah, and spake, saying, I will sing unto Jehovah, for he hath triumphed gloriously: The horse and his rider hath he thrown into the sea." — Exodus 15:1 (ASV)

Then sang Moses. Moses introduced this song not only as a testimony to his gratitude but also as a confirmation of the history. For the song he dictated to the Israelites was not about an unknown event; instead, he brought them forward as eyewitnesses, so that all ages might know that nothing up to this point had been written which had not been openly declared by 600,000 men, in addition to their wives and children.

Moses, therefore, set the example in keeping with his office, while the people, by singing with him, testified their approval in a way that allows no contradiction. For to whom could they have lied, since they were each other’s witnesses, and the song was heard by no strangers?

Moses seems to indicate their confidence by the Hebrew repetition, “they spoke, saying.” For this reason, too, their confession, declared by all of them, is more credible, because most of them soon after gave in to ingratitude: from this we gather that they only gave God glory under compulsion.

But, although Moses was the author of the song, he does not say “I will sing” for himself; instead, he prescribes to all what each individual should heartily do.