Taanis Chapter 3, Mishnah 7. This Mishnah deals with the question of which troubles warrant sounding the alarm even on Shabbos.
The text of the Mishnah:
"Al eilu matri'in b'Shabbos" - for the troubles listed below one may sound the alarm even on Shabbos.
On a simple reading this seems to mean that one is permitted to sound a shofar or trumpets on Shabbos. The Gemara, however, explains that the Mishnah is not referring to sounding an instrument, since on Shabbos we do not sound instruments, but rather to raising the alarm in a different sense: the very matter that is accomplished with a shofar during the week is accomplished on Shabbos through prayer - through the prayer of 'Aneinu', which is normally recited on a public fast day.
Which troubles warrant sounding the alarm:
"Ir shehikifuha goyim" - a city that has been surrounded by gentile marauders.
"O nahar" - a river that threatens to flood.
"O sefinah hametorefes bayam" - a ship that is sinking at sea.
The view of Rabbi Yosi:
Rabbi Yosi qualifies this and says: "L'ezrah, lo litz'akah" - sounding the alarm was permitted only in order to call for help, and not in order to cry out. That is, one does not cry out in prayer as the first Tanna held, but rather one raises a cry of alarm to people - "Save us, come and help" - so that they will come and assist in the difficult situation, whether it is a matter of invading armies, a threatening flood, or a sinking ship.
The view of Shimon HaTimni:
Shimon HaTimni adds: "Af al hadever" - one sounds the alarm for a plague as well. According to the first Tanna, sounding the alarm means reciting the prayer of 'Aneinu', whereas there are those who permit only crying out for help - if help is possible at all. It may well be, then, that Shimon HaTimni's statement is relevant only according to the view of the first Tanna.
The Mishnah concludes: "V'lo hodu lo Chachamim" - the Sages did not agree with him regarding sounding the alarm for a plague on Shabbos. And there are those who hold that in the opinion of the Sages a plague is not counted among the things for which one sounds the alarm even during the week.
In summary: In this Mishnah we learned which troubles warrant sounding the alarm even on Shabbos - a city surrounded by gentiles, a river that threatens to flood, and a ship sinking at sea - and that sounding the alarm on Shabbos is not done with an instrument but by reciting the prayer of 'Aneinu'. We also examined Rabbi Yosi's qualification, "for help, not for crying out", and the opinion of Shimon HaTimni, who adds a plague as well, with which the Sages did not agree.