Rashi on Numbers 15:34/רש"י על במדבר ט״ו:ל״ד

1כי 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).