Parashas Beshalach · Passuk 16:27
Chabad.org — English translation + Rashi
Torah Temimah points to Shabbos 118b (verified).
Sefaria — Exodus 16:27
Sefaria — Arachin 15a
E-Daf — Arachin 15a
English: Rabbi Yehudah says in Rav’s name that had Israel observed the first Shabbos, no nation would have ruled over them; the violation is juxtaposed with Amalek’s arrival.
On the daf / in context: See the cited source for the full section.
Derash on the verse.