Rashi on Numbers 25:4/רש"י על במדבר כ״ה:ד׳

1קח את כל ראשי העם TAKE ALL THE HEADS OF THE PEOPLE, to judge those who worshipped Peor (Sanhedrin 35a),

2והוקע אותם AND HANG THEM — those who worshipped it (not the heads of the people).

3והוקע — This is “hanging”, just as we find in the case of Saul’s sons, (II Samuel XXL 6) “And we will kill them by הוקעה unto the Lord”, and there (II Samuel vv. 12—13) hanging is distinctly mentioned (Sanhedrin 34b). It is true that idolatry (the sin committed here) is punishable by stoning, but all who were stoned were also hanged (Sanhedrin 45b).

4נגד השמש AGAINST THE SUN — i.e., in the sight of everyone. A Midrashic statement is: the sun made known who were the sinners, for the cloud rolled itself up from in front of him (the sinner) so that the sun shone upon him (Midrash Tanchuma, Balak 19).