Bamidbar 15:34

כי לא פרש מה יעשה לו [AND, THEY PLACED HIM IN WARD] BECAUSE IT WAS NOT EXPLAINED WHAT SHOULD BE DONE WITH HIM — i.e. what kind of death penalty he was to die had not yet been explained, but they were fully aware that one who desecrates the Sabbath is punishable by some death penalty (Sifrei Bamidbar 114; Sanhedrin 78b; cf. Rashi on Leviticus 24:12).

וַיַּנִּ֥יחוּ אֹת֖וֹ בַּמִּשְׁמָ֑ר כִּ֚י לֹ֣א פֹרַ֔שׁ מַה־יֵּעָשֶׂ֖ה לֽוֹ׃ {ס}        

And they put him in custody, because it was not declared what should be done to him.