במדבר ט״ו:ל״ב-ל״ד

ויהיו … במדבר וימצאו. בִּגְנוּתָן שֶׁל יִשְׂרָאֵל דִּבֵּר הַכָּתוּב, שֶׁלֹא שָׁמְרוּ אֶלָּא שַׁבָּת רִאשׁוֹנָה, וּבַשְּׁנִיָּה בָא זֶה וְחִלְּלָהּ (ספרי):

[Bnei Yisroel] were in the desert and they found. Scripture speaks disparagingly of the Israelites. They kept only the first Shabbos, then this person came and desecrated the second.

המצאים אתו מקשש. שֶׁהִתְרוּ בוֹ וְלֹא הִנִּיחַ מִלְּקוֹשֵׁשׁ, אַף מִשֶׁמְּצָאוּהוּ וְהִתְרוּ בוֹ (שם; סנהדרין מ"א):

Those who found him gathering. They warned him, but he did not desist from gathering, even after they found and warned him.

כי לא פרש מה יעשה לו. בְּאֵיזוֹ מִיתָה יָמוּת, אֲבָל יוֹדְעִים הָיוּ שֶׁמְּחַלֵּל שַׁבָּת בְּמִיתָה (שם ע"ח):

Since it was not specified, what was to be done with him. [They did not know] how to execute him, but they knew that someone who desecrates the Shabbos must be put to death.