Yevamos, Chapter 7, Mishnah 6. The Mishnah deals with testimony about a husband's death - testimony that has the power to permit his wife to remarry.
"Me'idin le'or hanner ule'or halevanah":
A person may testify that a man died and thereby permit his wife to remarry, and it is not necessary that he saw the body by daylight. Even if he saw it by candlelight at night, or by moonlight alone, he may still testify.
"Umasi'in al pi bas kol":
A woman may be permitted to remarry on the basis of testimony heard that her husband died, even if it came from nothing more than a bas kol. The intent here is not a 'bas kol' in the sense usually found in the words of Chazal, a voice from Heaven, but rather a sound or voice heard from a distance, in which someone called out "So-and-so died," even though the speaker was never seen. This is a voice with no body attached to it, and it is enough to permit her to remarry.
The Mishnah brings two incidents as examples:
"Ma'aseh be'echad she'amar al rosh har" - there was an incident with someone who stood on a mountaintop and called out loudly: "Ish peloni ben peloni mimakom peloni mes" - "So-and-so, son of so-and-so, from such-and-such a place, has died." "Halchu velo matz'u sham ish" - they went up to that mountaintop and found no one there, for this was like a voice without a body and they were unable to locate the speaker. "Vehisi'u es ishto" - and nevertheless they married off his wife.
"Veshuv ma'aseh beTzalmon" - another case occurred in a place called Tzalmon, "be'echad she'amar" - with someone who said about himself: "Ani ish peloni ben ish peloni, ushechani hanachash, va'ani mes" - he stated his own name and his father's name, related that a snake had bitten him, and said that he was about to die. "Vehalchu velo hikiru" - they went and saw the body but did not recognize the man, yet they had heard the name he stated. "Vehisi'u es ishto" - and they married off the wife of the man who bore that name, even though they were unable to identify the body precisely. It is enough that he testified about himself that he was that person and afterward died, in order to marry off his wife on the strength of this testimony.