Rashi on Numbers 26:3/רש"י על במדבר כ״ו:ג׳

1וידבר משה ואלעזר הכהן אתם AND MOSES AND ELEAZAR THE PRIEST SPOKE WITH THEM — They spoke with them (אתם; cf. 5:5) regarding this: that the Omnipresent had (given command to count them.

2לאמר SAYING they said to them (what they said to them was): You must be counted FROM TWENTY YEARS OLD AND UPWARDS.