Comentário sobre Êxodo 22:29

שבעת ימים יהיה עם אמו SEVEN DAYS IT SHALL BE WITH ITS DAM — This is an admonition for the priest (for the Israelite himself does not sacrifice the firstborn animal) that if he wishes to bring his offering of this at an earlier date (i.e. in the case of the Israelite having given it to him before the thirtieth or fiftieth day) he is not allowed to offer it before it is eight days old because it is then “short in time” (i. e. too young for sacrifice; cf. Leviticus 22:27).

כן תעשה לשורך לצאנך, that you shall give to Me the firstborn which is not a prematurely aborted fetus. Our sages have said (Shabbat 135) that “a human baby which remains alive for 30 days is considered viable, not in the category of נפל, prematurely aborted. In the case of animals the parallel period is eight days.”

This is a warning to the kohein. However, a person may consecrate his animal [as a sacrifice] even if it is only one day old. This warning, [i.e.,] negative commandment, is part of the positive commandment of “Seven days it shall remain. . .” [and thus is judged as a positive commandment].