TT #16 · 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 Pesachim 4a.
E-Daf — Pesachim 4a · Shas.org
English: The entire day is valid for circumcision, as it says, “on the eighth day he shall be circumcised,” although the zealous rise early to perform mitzvos.
Textual inference
Circumcision performed during the eighth day is valid throughout that day, with a preference for doing the mitzvah promptly.