Tractate Gittin - Chapter 7 - Mishnah 9 גיטין פרק ז׳ משנה ט׳

[If a husband says,] “If I do not come back within twelve months, write a get and give it to my wife”, and they wrote a get before twelve months had passed and gave it to her after, it is not a get. [If he said,] “Write a get and give it to my wife if I do not come back within twelve months”, and they wrote it before the twelve months had passed and gave it after, it is no get. Rabbi Yose says: like this is a get. If they wrote it after twelve months and delivered it after twelve months and he died, if the delivery of the get preceded his death the get is valid, but if his death preceded the delivery of the get it is not valid. If it is not known which was first, this is the woman about whom they said, “[She is] divorced and not divorced.”