Kesubos chapter 9, mishnah 1. This mishnah deals with cases in which a husband waives monetary rights that were granted to him by virtue of the marriage. The Gemara explains that the declaration he makes, or the document he writes to his wife waiving these rights, is made at the time of the betrothal - but before the marriage. For these rights come to him only after the marriage, and before they have come into his hands he stipulates that he does not want the marriage to grant them to him. In other words: he is not interested in receiving these rights at all. Such a condition takes effect, because a person may make a stipulation about something before it has reached his hands that he does not wish to receive.
However, once he has already received the rights, writing alone is not enough. In such a situation a kinyan is required - they must perform an actual act of acquisition, which expresses a firm decision to transfer away from him rights he has already acquired. Our mishnah deals with a case in which he writes to her alone, without a kinyan, and from this we learn that the writing took place before the marriage.
The first formulation: "Din u'devarim ein li b'nechasayich":
The mishnah opens: "HaKosev l'ishto: din u'devarim ein li b'nechasayich" - the husband writes to his wife, again before the marriage, that he has no claim or dealings with her property. This expression means that he does not intend to contest her property in any way, and that he has no share in her possessions. Even so, the mishnah rules: "Harei zeh ochel peiros b'chayeha" - he waived certain rights, but not the right to consume the produce of her melog property during her lifetime. And likewise "im meisah" - he did not waive the right of inheritance either, and if she dies he inherits from her.
The mishnah asks: "Im kein, lamah kasav lah din u'devarim ein li b'nechasayich?" - if these rights still remain in his hands, why did he declare that he has no share in her property? To this the mishnah answers: "B'machrah v'nasnah kayam" - this wording is effective in that if she sells her property or gives it away as a gift, the sale and the gift stand. Ordinarily her sale would have had no validity, but here the husband waived his right to claim that she may not sell this property.
The second formulation: "U'v'peiroseihen":
The mishnah continues and discusses one who wrote her a more explicit formulation: "Din u'devarim ein li b'nechasayich u'v'peiroseihen" - he added and explicitly excluded the produce as well. In such a case, "ein ochel peiros b'chayeha" - he does not consume the produce of her melog property, since he explicitly excluded it. But "im meisah" - he still inherits from her, because he did not exclude the right of inheritance.
Rabbi Yehudah says that even when he excluded the produce, "l'olam ochel peiri peiros" - he may still consume the produce of the produce. That is, he waived the produce, but if the wife took the produce and purchased land with it, and that land produced further produce - this is produce of produce, and his right to it remains, since he removed himself only from the direct produce of the melog property and not from the produce that comes by virtue of the produce. In his view, his right does not lapse until he writes her all the formulations explicitly:
"Din u'devarim ein li b'nechasayich" - I have no dealings with the property itself.
"U'v'peiroseihen" - and not with its produce either.
"U'v'peiri peiroseihen" - and not with the produce of the produce either.
"Ad olam" - forever.
Only once he writes this wording will he no longer have any right to the produce or to the produce of the produce.
The third formulation: "B'chayayich u'v'mosayich":
He went further and wrote to her: "Din u'devarim ein li b'nechasayich u'v'peiroseihen u'v'peiri peiroseihen b'chayayich u'v'mosayich" - that is, both during her lifetime and after her death. In this case, "ein ochel peiros b'chayeha", and not the produce of the produce either, "v'im meisah eino yorshah" - since he stated this explicitly.
Rabban Shimon ben Gamliel disagrees: "Im meisah yirashenah" - even though he stipulated explicitly that he wants no connection whatsoever to her property after her death, he still inherits from her. And the reason is: "Mipnei shehisnah al mah shekasuv baTorah" - for the Torah states "v'yarash osah," that the husband inherits from his wife. And since the law of inheritance is derived from the Torah itself, a person cannot waive it, for whoever stipulates against what is written in the Torah - "tena'o batel", his condition is void and has no validity at all.
The conclusion of the Gemara:
Although it appears from the mishnah that a husband's inheritance of his wife is from the Torah, the Gemara concludes that it is only rabbinic, and the verse is no more than a mere support. It is simply that the Sages reinforced their enactment, that a person inherits from his wife, to the point that they treated it as a Torah obligation.