레위기 12:3의 주석

וביום השמיני ימול, for by that time the blood of his mother has congealed [dry blood does not confer impurity. Ed.] As a result the baby has become ritually cleansed, no moist blood being attached to it.

וביום השמיני, and on the eighth day, etc. Why did the Torah have to instruct us to circumcise a male baby on the eighth day, something that has been legislated already in Genesis 17,12 prior to the birth of Isaac, even? If all the Torah wanted was to tell us that the rite of circumcision must be performed by day and not by night, or that the importance of circumcising the baby on the eighth day is so important that the Sabbath may be violated in order to fulfil this commandment on time (compare Shabbat 132), why did G'd not include this information in Genesis?