Nazir, Chapter 3, Mishnah 7. The Mishnah deals with a person who took a vow of nezirus, where two sets of witnesses testify about him with contradictory testimony regarding the number of nezirus vows he made.
The text of the Mishnah:
"Mi shehayu shtei kitei eidim me'idos alav" - two separate sets of witnesses testify about the vow of nezirus that he made, and their testimonies differ:
"Eilu me'idim shenazar shtayim" - these two witnesses testify that when he took the vow of nezirus, he vowed to be a nazir two times.
"Ve'eilu me'idim shenazar chamesh" - the other two witnesses testify that he vowed to be a nazir five times.
We are dealing, of course, with a case in which the man himself does not decide one way or the other: either he says that he does not remember what he said, or he says that he did not vow at all - and nevertheless there is testimony that he did vow.
The view of Beis Shammai:
"Beis Shammai omrim: nechlekah ha'eidus ve'ein kan nezirus" - according to Beis Shammai, the two testimonies contradict one another, and therefore they cancel each other out, as though there were no testimony here at all. Consequently there is no vow of nezirus here, and he need not fulfill one.
The view of Beis Hillel:
"U'Beis Hillel omrim: yesh biklal chamesh shtayim, sheyehei nazir shtayim" - Beis Hillel disagree: one set says five and the other says two, but both agree that he vowed at least two periods of nezirus. Since this point is common to both sets, there is here the testimony of two witnesses that he vowed to be a nazir at least twice, and therefore he is obligated in two periods of nezirus.
In summary: regarding a person about whom one set of witnesses testifies that he vowed two periods of nezirus and a second set testifies that he vowed five - Beis Shammai hold that the testimony is divided, the testimonies cancel one another, and there is no nezirus at all; Beis Hillel hold that the smaller number is included in the larger, and since both sets agree to a minimum of two periods of nezirus, he is a nazir twice.