Rashi on Exodus
ספר הברית THE BOOK OF THE COVENANT — the book which we have said contained the part of the Torah from בראשית till the “Giving of the Torah” including the Commandments that were given to them at Marah (Mekhilta d'Rabbi Yishmael 19:10:2; cf. Rashi on v. 4).
Sforno on Exodus
ספר הברית. The book in which G’d’s words had been recorded as well as the laws concerning which the covenant was concluded. Reference has to be made to this book in verse four of our chapter.
Rashbam on Exodus
ספר הברית, the book mentioned in verse four where Moses is reported as recording in writing.