Demai, Chapter Five, Mishnah Nine
This mishnah deals with tithing produce bought from a Gentile or from a Samaritan, a group of Israelites that split off from the rest of Israel some time during either the first or second Temple period.
1) They may give tithes from produce from a Jew for produce [bought] from a Gentile, from produce [bought] from a Gentile for produce from a Jew, from produce [bought] from a Jew for produce [bought] from Samaritans, and from produce [bought] from Samaritans for produce [bought] from [other] Samaritans.
2) Rabbi Eliezer prohibits [tithing] from produce [bought] from Samaritans for produce [bought] from [other] Samaritans.
Section one: Produce that a Gentile grows in the land of Israel is liable for the laws of tithing. The acquisition that the Gentile has on the land is not sufficient to make the produce exempt. Certainly Gentiles did not tithe their produce. Therefore, one who buys produce from a Gentile can use that produce to separate tithes from produce that was owned by a Jew and is known not to have been tithed. This is separating from certainly untithed produce for other certainly untithed produce, which is allowed.
Similarly, one may separate tithes from untithed produce that comes from a Jew and thereby exempt the produce that he bought from a Gentile.
Samaritans generally are thought by the rabbis to observe biblical law, but not to follow the instructions of the rabbis. According to our mishnah, Samaritans dont tithe the food that they sell, although they may tithe food that they eat. Therefore, their food is certainly untithed and can be treated with the same rules with which we treated the Gentiles food.
Section two: According to Rabbi Eliezer, some few Samaritans do tithe what they are going to sell. Therefore, one cannot separate tithes from produce bought from one Samaritan for produce bought from another Samaritan, lest one of the Samaritans be from the majority that dont tithe the produce they sell and the other be from the minority that does tithe that which they sell.