Exodus 15:25

שם 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). ושם 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.

וַיִּצְעַ֣ק אֶל־יְהֹוָ֗ה וַיּוֹרֵ֤הוּ יְהֹוָה֙ עֵ֔ץ וַיַּשְׁלֵךְ֙ אֶל־הַמַּ֔יִם וַֽיִּמְתְּק֖וּ הַמָּ֑יִם שָׁ֣ם שָׂ֥ם ל֛וֹ חֹ֥ק וּמִשְׁפָּ֖ט וְשָׁ֥ם נִסָּֽהוּ׃

And he cried to the Lord; and the Lord showed him a tree, which when he had cast into the waters, the waters were made sweet: there he made for them a statute and an ordinance, and there he tested them,