What is the makor for “osek b’mitzvah patur min hamitzvah”?
Answer:
See Sukkah 25a, where the Gemara brings the source of the halachah from 1) Keriyas Shema (somebody doing a mitzvah is exempt), and 2) Korban Pesach (it is permitted to become impure for the purpose of a mitzvah, even if this will mean missing the korban pesach).
See also Kovetz Shiurim Vol. 2, no. 32, concerning the logic of the principle, which he extracts from the halachah of Keriyas Shema: Only when a person is involved in matters of his own undertaking is he obligated in mitzvos; when a person is involved in matters of mitzvos (even rabbinic), he is not obligated in other mitzvos.
