Mishnayos Gittin Perek 7 Mishnah 3

If one says to his wife: This is your bill of divorce if I die, or: This is your bill of divorce if I die from this illness, or: This is your bill of divorce after my death, then it is as if he said nothing, since a bill of divorce is valid only if it takes effect before the husband’s death. But if the husband said to his wife: This is your bill of divorce from today if I die, or: This is your bill of divorce from now if I die, then this is a valid bill of divorce, because once he dies, the bill of divorce retroactively applies from when he made this statement. If the husband says to his wife: This is your bill of divorce from today and after my death, then it is uncertain whether his primary intention was for the bill of divorce to take effect that day, in which case it is a valid bill of divorce, or if his primary intention was that it should take effect after his death and is therefore not valid. The halakha is that there is uncertainty whether it is a valid bill of divorce or not a valid bill of divorce. And if he dies without children his wife must perform ḥalitza, since perhaps the bill of divorce is not valid and she is bound by the levirate bond and may not remarry without first performing ḥalitza. But she may not enter into levirate marriage, since perhaps the bill of divorce is valid, and it is prohibited for a divorcée to marry her brother-in-law. If he said: This is your bill of divorce from today if I die from this illness, and he recovered, and he arose and walked in the market, but then became ill again and died, the court assesses him. If he died because of the first illness then this is a valid bill of divorce, as his conditional statement was fulfilled, but if not, i.e., if he was cured from the first illness and died from another illness, then it is not a valid bill of divorce.