Yosef Karo, Shulchan Aruch — Yoreh Deah 266
“ About the measuring of twilight, the Tannaim and the Amoraim disagree at the end of the chapter “Bameh Madlikin” [the second chapter of Shabbat] . The Ba’al HaIttur wrote that there is doubt for us regarding whom the halakhah is like; therefore, if the head of the child was coming out from sunset, there is doubt until twilight according to R. Yose is completed, and the child is circumcised on the 10th day. If the child was born Saturday night from sunset on, we follow the stringency of Rabba. Seif 13 – If a Jew has born to him a son by a non-Jew, he does not circumcise him on Shabbat. ”
