עירובי תחומין א

When one places an eruv — that is, food for two meals — one is permitted to walk from the place of the eruv two thousand amos [cubits] in every direction. It emerges that walking on Shabbos depends on the meals of Shabbos. And this is explained in the words of Rabbainu, of blessed memory, in the teaching that discusses machlo’kes [dispute], on the verse “When evildoers rise against me, my ears shall hear” (siman 277). It is explained there that the main walking of Shabbos depends on the eating of Shabbos, in the aspect of “dok b’chakay v’sishkach b’nagray” [check the teeth and you will find in the legs] — see there well. And therefore our Sages, of blessed memory, established that the place of the meal determines the location from which one may walk the t’chum Shabbos [Shabbos boundary]. For the main walking depends on the meal, as above — see there.

And therefore immediately after Purim one reads Parshas Parah — "This is the statute of the Torah" — as stated above, and understand this.