Sforno on Numbers 26:4

לאמר מבן עשרים שנה ומעלה. משה ואלעזר אמרו לישראל שיאמרו ויגידו כל פרטיהם שהם בני עשרים: לאמר מבן עשרים שנה ומעלה; Moses and Eleazar told the Israelites to inform them with a list of all the details of the males who had attained the age of 20. This was parallel to what we were told in Numbers chapter 1 at the previous census, hence the words “as G’d had commanded,” etc.; if these words had referred to what occurred now there would have been no need for this phrase. At that time G’d’s instructions had included the words: “according to their families, the houses of their fathers,” i.e. the co-operation of each family in the census had been requested.