Related for Chullin 7:32
פשיטא
R'Simeon B'Gamaliel says: Whatever law the Cutheans have adopted, they are very strict in the observance thereof, more so than Israelites'.
Tosefta Pesachim
The matzah of the Samaritans is permitted, and a person fulfills his obligation on Passover [with it]. But Rabbi Eliezer forbids it because [according to him,] they are not learned in the details of the mitzvah. Rabban Shimon ben Gamaliel says, every mitzvah that the Samaritans accepted upon themselves -- they are more scrupulous in it[s observance] than a Jew.
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