Gemara:

Bava Batra,  Bava Metzia 

Description

Explores the nature of the takanas chachamim that the malveh may not collect karka if it was a milveh bi'al peh. Also explains the Ramabam who says in one place that "shibuda di'oraisa" and in another place seems to say "shibuda lav di'oraisa" using a yesod of the Rogochover Gaon explicated in a different context. Quite exciting if you like lomdus.