Reference for Pesachim 137:3
אמר לו ר' עקיבא דמה לי אם דחו מכשירי מצוה שלאחר שחיטה את השבת שהרי דחתה שחיטה את השבת תאמר ידחו מכשירי מצוה שלפני שחיטה את השבת שלא דחתה שחיטה את השבת דבר אחר שמא ימצא זבח פסול ונמצא מחלל את השבת למפרע
be studied by day and by night, heaven and earth would not enjoy permanence. How then could R. Shesheth take such a selfish view of his studies? R. AKIBA ANSWERED AND SAID: LET HAZAAH PROVE IT etc. It was taught, R. Eliezer said to him: 'Akiba, you have refuted me by shechitah,' by shechitah shall be his death!' Said he to him 'Master, do not deny me at the time of argument: I have thus received [the law] from you. [vis.] haza'ah is a shebuth and does not override the Sabbath.' Then since he himself had taught it to him, what is the reason that he retracted? - Said 'Ulla: When R. Eliezer taught it to him it was concerning haza'ah for [the sake of] terumah, since terumah itself does not override the Sabbath; [and] R. Akiba too, when he refuted him refuted him by haza'ah for [the sake of] terumah, which is [likewise] a religious duty and is [usually forbidden] as a shebuth; but he [R. Eliezer] thought that he was refuting him by haza'ah for the Passover sacrifice. <br>