Tractate Tahorot - Chapter 5 - Mishnah 9 טהרות פרק ה׳ משנה ט׳

If one witness says, "You have become unclean," but he says, "I have not become unclean," he is regarded as clean. If two witnesses say, "You have become unclean," and he says, "I have not become unclean," Rabbi Meir says: he is unclean. But the sages say: he may be believed on his own evidence. If one witness says, "You have become unclean," and two witnesses say, "He has not become unclean," whether in a private domain or in a public domain, he is regarded as clean. If two witnesses say, "He has become unclean’, and one witness says, ‘"He has not become unclean," whether in a private domain or in a public domain, he is regarded as unclean. If one witness says, "He has become unclean," and another says, "He has not become unclean," or if one woman says, "He has become unclean’, and another woman says, "He has not become unclean," he is regarded as unclean if in the private domain, but if in a public domain he is regarded as clean.