Ketubot 13B

או לחורבה ⁠— ואמרו לה: מה טיבו של איש זה? כהן הוא, ובן אחי אבא הוא. רבן גמליאל ורבי אליעזר אומרים: נאמנת, רבי יהושע אומר: לא מפיה אנו חיין, אלא הרי זו בחזקת בעולה לנתין ולממזר, עד שתביא ראיה לדבריה.

or into a ruin, which is typically located outside the city, and if a man and woman meet there it is presumably in order to have sex, and people said to her: What is the nature of this man with whom you secluded? She said to them: He is a priest, and he is the son of my father’s brother, a respectable person of impeccable lineage. Rabban Gamliel and R' Eliezer say: She is deemed credible. R' Yehoshua says: It is not based on the statement emerging from her mouth that we conduct our lives; rather, she assumes the presumptive status of one who had sex with a Gibeonite or with a mamzer, men of flawed lineage who disqualify her from marrying a priest, until she brings proof supporting her statement.

בשלמא לזעירי ⁠— היינו דקתני תרתי: לסתר או לחורבה. אלא לרב אסי דאמר נבעלה, תרתי למה לי? חדא קתני: לסתר דחורבה.

Granted, according to Ze’eiri, who said that speaking means that she secluded herself, that is the reason that the tanna teaches two cases where there is merely concern: The case of: Into seclusion, where there is lesser concern that she had sex, and the case of: Into a ruin, where there is greater concern. However, according to Rav Asi, who said that speaking means that she had sex, and only in that case is she not deemed credible according to R' Yehoshua, why do I need two cases? The Talmud answers: According to Rav Asi, the tanna is teaching one case: If people saw a woman enter with one man into the seclusion of a ruin, where the likelihood is that that they entered to have sex.