Pesachim, Chapter 4, Mishnah 1. This mishnah deals with the prohibition against doing work on the eve of Pesach, during the hours when the Pesach offering is normally brought - from the afternoon onward. The underlying idea: it is not fitting for a person to be preoccupied with his work at the very time when he ought to be occupied with bringing the Pesach offering, and this applies even if someone else is offering it on his behalf.
A law that depends on local custom:
The question the mishnah raises is whether the Sages extended this law to the entire day, and the answer depends on custom: "Makom shenahagu la'asos melachah b'arvei Pesachim ad chatzos - osin; makom shenahagu shelo la'asos - ein osin" - in a place where the custom is to do work on the eve of Pesach until midday, one is permitted to do work; and in a place where the custom is not to do work, he may not do it, but must follow the local custom. The general rule: a person follows the custom of his own place.
One who travels from place to place:
From here the mishnah raises a further question: what is the law for someone who changes his location? Whether he went from a place where work is done on the morning of the eve of Pesach to a place where it is not done, or from a place where work is not done to a place where it is done - we impose upon him the stringencies of both places: the stringencies of the place he left, and the stringencies of the place he came to.
The mishnah concludes that "al yeshaneh adam" - a person should not deviate from the custom of the place where he finds himself, because there is no reason for the matter to lead to friction through his not following the practice of that region.
The dispute between Abaye and Rava in explaining the closing clause:
In the Gemara, Abaye and Rava disagree about the purpose of this final statement:
According to Abaye: the statement refers to someone who went from a place where work is done to a place where it is not done. We tell him: do not deviate from the custom of the place you have arrived at, and do not do work there, since work is not done there, and otherwise the matter will lead to friction. But one who goes from a place where work is not done to a place where it is done retains the stringency of the place he came from and does not do work.
According to Rava: the closing line deals specifically with someone who went from a place where work is not done to a place where it is done, and it comes to teach us that even though he is now in a place whose custom is to do work and he is not doing it, he need not be concerned. For the entire purpose of this law is that he should not cause friction, and his refraining from work is neither a sign of dispute nor a cause of one, since many people simply have no work and nothing to do. It turns out that not doing work gives no indication at all that he disagrees with the local custom.
One important note about this mishnah: this entire law, that he must keep the custom of the place he came from, was stated specifically where he intends to return there. But if he has moved to the new place permanently, he follows the custom of that new place in which he finds himself entirely.