TT #17 · 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 Megillah 20a.
E-Daf — Megillah 20a · Shas.org
English: Circumcision is not performed until sunrise, as it says, “on the eighth day he shall be circumcised.”
Textual inference
The mitzvah's designation as a daytime act requires waiting until sunrise.