Negative commandment · Lo Ta’aseh #95 of 613
Not to offer animals with disqualifying physical blemishes of a temporary nature as sacrifices
שלא להקריב בעל מום עובר
The verse · Devarim 17:1
לֹא־תִזְבַּח לַה׳ אֱלֹהֶיךָ שׁוֹר וָשֶׂה אֲשֶׁר יִהְיֶה בוֹ מוּם כֹּל דָּבָר רָע כִּי תוֹעֲבַת ה׳ אֱלֹהֶיךָ הוּא
“Thou shalt not sacrifice to the Lord thy God any bullock, or sheep, in which is a blemish, or anything evil: for that is an abomination to the Lord thy God.”
The Rambam’s words
Not to offer animals with disqualifying physical blemishes of a temporary nature as sacrifices, as [Deuteronomy 17:1] states: "Do not sacrifice to G-d, your Lord, an ox or a sheep that has a blemish." This refers to a blemish of a temporary nature.
שלא להקריב בעל מום עובר, שנאמר "לא תזבח לה' אלוהיך שור ושה, אשר יהיה בו מום" (דברים יז,א); זה הוא מום עובר.
Why · Sefer HaChinuch #494
To not offer a sacrifice of an animal with a temporary blemish
“It is from the roots of the commandment [that it is] since most activities of people are pleasing to the hearts of their observers based on the importance of the ones doing them.”
Who & when
“And this prohibition is practiced at the time of the [Temple], as it was then in our hands to offer sacrifices.”
Where in the Mishneh Torah
Hilchot Things Forbidden on the Altar הלכות איסורי המזבח