The 321st prohibition is that we are forbidden to travel [even by foot] on Shabbos.
The source of this commandment is G‑d's statement1, "No man may leave his place on the seventh day."
Oral Tradition defines limit of "travel" as 2000 amos2 beyond the city limit; one may not walk even a single amoh more. However, one may walk within 2000 amos from the city limit in any direction.3
The Mechilta says: " 'No man may leave his place,' — this means beyond 2000 amos."
Our Sages said in tractate Eruvin,4 "For violating the prohibition of the Shabbos limit, one receives lashes by Biblical ordinance."5
The details of this commandment are discussed in that same tractate.
