TT #21 · Parshas Tazria · Aliyah 1 · Passuk 3
English: On the eighth day the flesh of his foreskin shall be circumcised.
Chabad.org — Chumash, English translation + Rashi for Vayikra 12:3
Torah Temimah cites Yerushalmi Rosh Hashanah 1:1.
Sefaria — Yerushalmi Rosh Hashanah, Chapter 1
English: Rabbi Chaggai asked Rabbi Yose: The verse says, “On the eighth day he shall be circumcised.” If the eighth day passed and he was not circumcised, what is the rule? Rabbi Yose answered with the verse, “You shall not delay to fulfill [your vow].” That rule applies to an obligation that can be made up later; it excludes circumcision on the eighth day, because that day itself cannot be made up.
Analogy from another verse
Missing the eighth-day timing is not treated as a violation of the specific prohibition against delaying a vow, because the eighth-day obligation itself cannot be recreated.