Demai, Chapter 3, Mishnah 6. This mishnah introduces us to the concept of the "mother-in-law" - a concept that needs no introduction, since we all know it. Even so, this mother-in-law is no subject for mother-in-law jokes, because she is looking out for her son-in-law's good.
The law of one who gives his food to his mother-in-law:
"HaNosen laChamoso - me'aser es shehu machnis v'es shehu motzi, mipnei shehi chashudah lachalof es hamiskalkel" - one who gives his food to his mother-in-law, and she is not among those meticulous about separating tithes, must separate tithes on the food he brings in to her, and not only on what he takes back from her, because she is suspected of making a switch.
But the switch we are concerned about is not one to his detriment, but rather precisely for his benefit: part of his food is liable to spoil, and she does not want her son-in-law to have spoiled food but rather good food. She is therefore liable to swap his food for part of her own food, which is of better quality. And because this interest in his benefit stands before her, he must be concerned that she will take his food on account of its inferiority, and he must see to separating tithes even before he gives her his food - even though by law she is supposed to return his own food to him. All this is for the case in which she swapped it for part of her own food and he cannot detect it, so that he ends up holding her food, from which tithes have already been separated.
Rabbi Yehudah's reasoning:
Rabbi Yehudah explains: "Rotzah hi b'sakanas bitah u'voshah m'chasanah" - she desires her daughter's welfare and success, and she is embarrassed before her son-in-law and wants to make sure he receives what is fitting for him. And since it is Rabbi Yehudah who provides the reasoning behind the law, it is evident that the first part of the mishnah is his statement as well.
Rabbi Yehudah concedes regarding produce of the Sabbatical year:
"Modeh Rabbi Yehudah b'nosen laChamoso sheviis, she'einah chashudah l'ha'achil es bitah sheviis" - when he gives his mother-in-law produce of the Sabbatical year, there is no concern that she will switch it and feed her daughter and son-in-law Sabbatical-year food that is not permitted to be eaten, such as fruit that was guarded during the Sabbatical year. The reason: in Rabbi Yehudah's view, the Sabbatical year is something that even the unlearned are more meticulous about, and they regard it as a significant mitzvah and a significant transgression, and therefore there is no concern that they would hand over the wrong item during the Sabbatical year. The suspicion that exists regarding tithes does not exist regarding the Sabbatical year.
In summary: In this mishnah we learned that one who gives his food to his mother-in-law who is not meticulous about tithes separates tithes both on what he brings in and on what he takes out, because of the concern that she will swap out what is spoiling out of her concern for his benefit. Rabbi Yehudah explains this by her desire for her daughter's welfare and her embarrassment before her son-in-law, and he concedes that with produce of the Sabbatical year there is no concern about a switch, since even the unlearned are meticulous about the Sabbatical year.