Related for Chullin 7:37
לא בקיאי בשימור
If this is so, why does R'Simeon B'Gamaliel say: 'Whatever precept the Cutheans have adopted'?
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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