The Credibility of a Solitary Witness — Eruvin 58b
תלמוד בבלי מסכת עירובין דף נח/ב
ואפילו עבד אפילו שפחה נאמנין לומר עד כאן תחום שבת שלא אמרו חכמים את הדבר להחמיר אלא להקל:
תוספות עירובין דף נט/א
ותחומין דרבנן - משמע אי הוה דאורייתא לא מהימני ואע"ג דמעשה בכל יום שמאמינין לנשים בשחיטה וניקור
ולתרום חלה היינו משום דהוי בידה קודם שנעשית השחיטה הוי בידה לשחוט אבל לתחומין לא הוי כלל בידה
The mishnah tells us that even slaves are believed to testify as to the extent of the Techum Shabbos of a locality.
The Gemara (below, 59a) tells that the reason why we are lenient to such an extent is because the 2000 amos of Techum Shabbos are a rabbinic enactment. The obvious inference is that if the 2000 amah limit had been Torah law, we would not be allowed to accept the testimony of the slaves.
For that matter, assume the Tosafos (ad loc., d.h. U'techumin), we would not be allowed to accept the testimony of any person whose testimony is not acceptable in a court of law. Thus, we would not be allowed to accept the testimony of a woman. Yet, ask the Tosafos, we regularly believe women concerning more severe areas of Torah law — viz., kashrus and niddah, etc. — why not in connection with Techum Shabbos?
The Tosafos respons that we only accept the testimony of women and other witnesses who cannot testify in a court of law when they testify concerning a matter that is, or at least was, under her control (בידה) at some point. For example, at some point in the past a woman could have exercised her control on this piece of meat (while It was still a part of a live cow) and slaughtered it properly. Or, at some point, either in the past or the present, a woman who was a niddah could have exercise her control of her actions and immersed in a mikvah. The extent of the Techum Shabbos, on the other hand, is an objective matter of measurement, not subject to her control, and therefore a matter in which we cannot credit her testimony.
The Beis HaLevi (Teshuvos 2:37.4) derives from this discussion a fundamental stringency regarding the credibility of any solitary witness — even a male, since the testimony of a solitary man is also not accepted in a court of law. To whit, a solitary witness of a any gender or background is never credited unless the matter of his or her testimony is one that was at one time subject to his or her control. From our case we see that this principle applies even to a case in which there is no a priori assumption of a prohibition (חזקת איסור), as there is no a priori assumption that a specific place near a city is beyond its Techum Shabbos.