Rashi on Exodus 15:25/רש"י על שמות ט״ו:כ״ה

1שם שם לו HERE HE MADE FOR THEM [A STATUTE AND AN ORDINANCE) — At Marah He gave them a few sections of the Torah in order that they might engage in the study thereof; viz., the sections containing the command regarding the sabbath, the red heifer and the administration of justice (Mekhilta; Sanhedrin 56b).

2ושם נסהו AND THERE HE TRIED THEM (lit., He tried him) — “him” (singular) means the people; and He saw their stubbornness: that they did not consult with Moses in a respectful fashion: “Entreat mercy for us that we may have water to drink” — but that they murmured.