Rashi on Leviticus 24:12/רש"י על ויקרא כ״ד:י״ב

1ויניחהו AND THEY PLACED HIM [IN WARD] — him by himself — and they did not place the man who gathered sticks on the Sabbath day with him (Numbers 15:34), because both of whom happened to be (i. e. to commit an offense) at the same period, but they knew that the stick-gatherer was to be punished with the death-penalty, only it had not been explained to them by what kind of death he was to be punished — it is for this reason that it is stated in his case (Numbers 15:33) “[and they placed him in ward] because it was not explained what should be done to him”. In the case of the blasphemer, however, it states “[and they placed him in ward] that [the proper penalty] might be shown to them”; this was because they did not know whether he is at all liable to the death-penalty or not (Sifra, Emor, Section 14 5; Sanhedrin 78b).