Shabbos, Chapter 1, Mishnah 6. This Mishnah continues the central theme of the chapter: the dispute between Beis Shammai and Beis Hillel as to whether a person is obligated to ensure that the objects he owns - even the inanimate objects and vessels he owns - do not perform labor on Shabbos.
The text of the Mishnah:
"Beis Shammai omrim: ein nosnin unin shel pishtan l'soch hatanur" - one may not place bundles of flax into the oven. It was common practice to place them in the oven in order to heat them, as part of the process of preparing flax.
"Ela k'dei sheyavhilu mib'od yom" - one may not do so unless the flax will have time to heat up completely while it is still Erev Shabbos.
"V'lo es hatzemer l'yorah" - similarly, one may not place wool into a cauldron or pot that is on the fire together with the materials needed to dye it, since it is dyed inside that very cauldron that sits on the fire.
"Ela k'dei sheyiklot ha'ayin" - one may not do so unless there is enough time for the wool to absorb the appearance, meaning the color, before Shabbos begins.
"U'Beis Hillel matirin" - Beis Hillel permit it, since all that is involved here is allowing a person's vessels and objects to perform labor on Shabbos.
The qualifications of the Gemara:
The Gemara notes with regard to the dyeing, and apparently this would apply to the oven as well, that we are dealing with a pot that is no longer on the fire. For if it is on the fire, as we will see later in the tractate, there is a concern that a person may come to stir the coals in order to speed up the process.
The Gemara further says that we must be dealing with a sealed pot, one that was sealed shut so that it cannot be opened. Were it not sealed, there would be a concern that the person might forget and open the pot and stir it, thereby becoming liable for the labor of cooking on Shabbos. To avoid all of this, both qualifications are required together, even according to Beis Hillel:
That the pot is not standing on the fire.
That the cover is sealed, so that he will not come to stir it.
A further example of the resting of vessels:
Beis Shammai bring an additional example of the matter of the resting of vessels, that one may not allow his vessels to perform labor:
"Ein porsin metzudos chayah v'ofos v'dagim" - one may not spread traps for wild animals, for birds or for fish.
"Ela k'dei sheyatzudu mib'od yom" - unless the matter is arranged in such a way that they will be caught in them before Shabbos.
"U'Beis Hillel matirin" - Beis Hillel permit it, and even if the trapping takes place on Shabbos itself, the person is not performing the labor; rather, his vessels are performing it, and in their view there is no prohibition in this.
In summary: This Mishnah continues the dispute between Beis Shammai and Beis Hillel regarding the resting of vessels - with flax in the oven, wool in the cauldron, and traps for animals, birds and fish. Beis Shammai require that the labor be completed, or at the very least begin to take effect, while it is still day, whereas Beis Hillel permit it even though the labor continues into Shabbos, since it is the vessels that are acting and not the person. The Gemara added two qualifications: that the pot not be on the fire, lest he stir the coals, and that it be covered in a sealed manner, lest he come to stir it and become liable for cooking.