John Gill Commentary Numbers 15:34

"And they put him in ward, because it had not been declared what should be done to him." — Numbers 15:34 (ASV)

And they put him in ward
In a certain prison in the camp; perhaps the same in which the blasphemer was put, (Leviticus 24:12); and for much the same reason:

because it was not declared what should be done to him :
that is, what kind of death he should die, as Jarchi and Aben Ezra; it had been before declared that the sabbath breaker should die, but not what death he should die, (Exodus 31:15) (35:2); though some think it was a matter of doubt whether gathering of sticks was a breach of the sabbath, or at least such a breach of it as required death; and the answer of the Lord seems to confirm this sense, as follows.