Yevamot 42A

לאחר שלשה ⁠— אין צריכה להמתין שלשה חדשים.

If they performed the ḥalitza after 3 months, she does not need to wait 3 months and may marry immediately.

הוי (ה)שלשה חדשים שאמרו, משעת מיתת הבעל, ולא משעת חליצת היבם.

The Talmud infers from the latter clause of the baraita: It must be that the 3 months that are stated throughout the baraita are counted from the time of the husband’s death and not from the time of the ḥalitza of the yavam.

מאי שנא מגט, דרב אמר: משעת נתינה, ושמואל אמר: משעת כתיבה!

The Talmud asks: In what way is this case different from the case of a bill of divorce, where Rav said that the 3 months are counted from the time of the giving of the bill of divorce, and Shmuel said that the count is from the time of the writing of the bill. If a couple is secluded together after the bill of divorce is written, the bill of divorce is invalid. Therefore, there is no concern that they were secluded from that time. This is why Shmuel holds that the 3 months are counted from the writing. Rav apparently assumes that even so, the count always begins from the formal end of the marriage and not from the point from which there was no possibility of her becoming pregnant. Why, then, does the baraita not rule also in the case when a woman happens for levirate marriage, that the count should begin from the point of ḥalitza, since the marriage is only fully severed at that point?

אמר רבא, קל וחומר: איסור כרת התרת, איסור לאו ⁠— לא כל שכן.

Rava said: With regard to a yevama, all agree that the count begins from the time of her husband’s death. This can be derived through an a fortiori inference, as follows: If a prohibition that entails karet, i.e., the prohibition against having sex with one’s brother’s wife in the event she is pregnant and not subject to levirate marriage, you have permitted after 3 months from the husband’s death, then in the case of a standard negative prohibition, such as the prohibition against having sex with a woman within 3 months of the death of her previous husband, is it not all the more so clear that she should be permitted to remarry after 3 months from her husband’s death? Therefore, even Rav agrees that in this case, the count begins from the husband’s death.