I heard that there is a letter from the Rebbe Rashab (Chabad) that one shouldn't eat chametz on Achron Shel Pesach when they are in Eretz Yisrael (even if they are keeping only one-day Yom Tov.) I understood perhaps it was directed specifically towards someone that was really from Chutz L'Aretz and was keeping one-day Yom Tov for whatever reason.
Has anyone seen or heard of such a letter? What's the source for this?
(I heard this originally b'shem a big Talmud Chacham that has a connection to Chabad, however I was never able to ask him where the makor is.)