Talmud for Shabbat 184:13

“In truth.” Rebbi Eliezer said, any place where “in truth” was stated it is practice going back to Moses on Sinai⁴⁸Babli 92b. In the two other quotes of this statement, Kilaim 2:2 Note 36, Terumot 2:1 Note 16, the tradent is R. Eleazar..