Sforno on Exodus 15:19/ספורנו על שמות ט״ו:י״ט

1כי בא סוס פרעה, the opening words of this song,אז ישיר משה had been inspired by the spectacle of Pharaoh with his chariot, his horses and its riders drowning in the sea at the time when the Israelites, by contrast, were taking their time walking through the same sea on dry ground. ביבשה בתוך הים, they began the song of thanksgiving even before all had emerged from the sea to the shore.