Reference for Shabbat 270:7
(אמר רבא) בשלמא לר' חמא משכחת לה יליד בית נימול לא' יליד בית נימול לח' מקנת כסף נימול לא' ומקנת כסף נימול לח' ילדה ואח"כ הטבילה זהו יליד בית שנימול לאחד הטבילה ואח"כ ילדה זהו יליד בית שנימול לח'
that is [a slave] born in his [master's] house who is circumcised on the first day; if she has a ritual bath and then gives birth, that is [a slave] born in his [master's] house who is circumcised at eight days. But the first Tanna allows no distinction between one who [first] has a ritual bath and then gives birth and one who gives birth and then has a ritual bath, so that though his mother is not defiled through her confinement he is circumcised on the eighth day.<span class="x" onmousemove="('comment',' These laws centre on Gen. XVII, 12, 13: And he that is eight days old shall be circumcised among you, every male throughout your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed (v. 12). He that is born in thy house, and he that is bought with thy money, must needs be circumcised (v. 13). Whereas v. 12 specifies circumcision for the eighth day, v. 13 does not, which implies at the earliest possible moment. Now it is logical that v. 12 refers to a slave who is as like as possible to a full Jew, that being the implication of 'among you', intimating those that are similar to you. viz., one born in his master's house after he was purchased. i.e., his mother was bearing him when she was bought; whilst v. 13 applies to a slave who is unlike a full Jew, vi., he was already born before he was bought. R. Hama draws this distinction: If his mother has a ritual bath, whereby she formally becomes a Jewish-owned slave in that she is bound to observe all the laws incumbent upon Jewesses in general, so that her confinement renders her unclean just like a Jewess, and then she gives birth, the infant is circumcised on the eighth day. But otherwise the infant is not like a Jewish-born child, and is circumcised on the first day. But the first Tanna ignores this distinction: thus R. Assi's ruling is a matter of controversy between the first Tanna and R. Hama. ');"><sup>7</sup></span>