Parashas Yisro · Passuk 19:10
Chabad.org — English translation + Rashi
Torah Temimah points to Shabbos 87a (verified).
English: The Gemara records Moses adding one day on his own, deriving it from היום ומחר by treating “today” like “tomorrow,” whose night is included.
In context: See the cited source for the full section.
Derash on the verse.