Exodus 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). ביום השמיני תתנו לי ON THE EIGHTH DAY THOU SHALT GIVE IT ME — One might think that it is obligatory to offer it on that day! Scripture, however, uses here the words “eighth day” and uses them again later on, (Leviticus 22:27) “and from the eighth day and henceforth it shall be favourably accepted”, in order to suggest an analogy. Now what is the meaning of the “eighth day” mentioned in the latter passage? It intends to declare the first-born fit for sacrifice from the eighth day and henceforth (i. e. it is intended to fix the earliest possible time on which it may be sacrificed). This, too, in the purpose of the word שמיני used here: to declare it fit for sacrifice from the eighth day and henceforth. The text therefore implies: “on the eighth day thou mayest give it to Me” (and not “thou shalt give it to Me”) (Mekhilta).

כֵּֽן־תַּעֲשֶׂ֥ה לְשֹׁרְךָ֖ לְצֹאנֶ֑ךָ שִׁבְעַ֤ת יָמִים֙ יִהְיֶ֣ה עִם־אִמּ֔וֹ בַּיּ֥וֹם הַשְּׁמִינִ֖י תִּתְּנוֹ־לִֽי׃

In like manner shalt thou do with thy oxen, and with thy sheep: seven days it shall be with his dam; on the eighth day thou shalt give it me.