משנה אויף במדבר 9:3

Rabbi Eliezer said: is it not logical: if slaughtering, which is [usually forbidden] as a labor, overrides Shabbat, shouldn’t these, which are [only forbidden] as mandated rest (shevut), override Shabbat? Rabbi Joshua said to him: let the festival prove this, for they permitted labor [on the festival] and forbade [activities forbidden because of] shevut. Rabbi Eliezer said to him: what is this, Joshua? What proof is a voluntary act in respect of a commandment! Rabbi Akiva answered and said: let sprinkling [purificatory waters] prove it, which is [performed] because it is a commandment and is [forbidden only] as a shevut, yet it does not override Shabbat; so you too, do not wonder at these, that though they are [required] on account of a commandment and are [forbidden only] as shevut, yet they do not override Shabbat. Rabbi Eliezer said to him: but in respect of that I am arguing: if slaughtering, which is a labor, overrides Shabbat, is it not logical that sprinkling, which is [only] a shevut, should override Shabbat! Rabbi Akiva said to him: or the opposite: if sprinkling, which is [forbidden] as a shevut, does not override Shabbat, then slaughtering, which is [normally forbidden] on account of labor, is it not logical that it should not override Shabbat. Rabbi Eliezer said to him: Akiva! You are uprooting what is written in the Torah, “at twilight, offer it at its set time” (Numbers 9:3), both on week-days and on Shabbat. He said to him: master, give me an appointed time for these as there is an appointed season for slaughtering! Rabbi Akiva stated a general rule: work which could be done on the eve of Shabbat does not override Shabbat; slaughtering, which could not be done on the eve of Shabbat, does override Shabbat.