172 Yevamot 15/3-4

(3) Beit Shammai say: [The woman who reports her husband's death] may marry and collect her Ketubah [a monetary settlement payable to a woman upon divorce or the death of her husband]. Beit Hillel say: She may marry but does not collect her Ketubah. Beit Shammai said to them: You allow the stringent Ervah [forbidden union], will you not allow the lenient monetary issue? Beit Hillel said to them: We find that brothers do not inherit based on her word. Beit Shammai said to them: Alas, we learn from the text of the Ketubah that he writes for her, "That if you marry somebody else you may collect what is written for you". And Beit Hillel retracted to teach according to Beit Shammai.

(4) All people are trusted to testify to her except for her mother-in-law, and her mother-in-law's daughter, and her rival, and her Yevama [her brother-in-law's wife], and her husband's daughter. Why is [reporting] a bill of divorce different than [reporting] a death? Because the writing [in the bill of divorce] proves [the divorce]. [If a single] witness states, "He died", and she married, and a different [witness] came and said, "He did not die", she does not go out. [If a single] witness states, "He died", and two [other witnesses] state, "He did not die", even if she has married - she goes out. [If] two witnesses state, "He died", and a [single] witness states "He did not die", even though she has not married, she may marry.