Reference for Shabbat 270:9
אלא לת"ק בשלמא כולהו משכחת להו אלא יליד בית נימול לאחד היכי משכחת לה
As for R. Hama, it is well: we find [a slave] born in his [master's] house who is circumcised on the first day, and one who is circumcised on the eighth day; one bought with money who is circumcised on the first day, and one bought with money who is circumcised on the eighth day. [Thus:] if she gives birth and then has a ritual bath, 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 the house who is circumcised on the eighth [day].<span class="x" onmousemove="('comment',' Both of these refer to a slave who conceived in her master's house, so that the infant is not 'bought with money'. ');"><sup>9</sup></span>