Related for Chullin 274:26
תרומה עין יפה אחד מארבעים
but by Rabbinic enactment it is one fortieth part.
Tosefta Peah
These are the things that have no set amount: the peah, the bikkurim, the r'iyah, charity, and learning Torah. Peah has a minimum amount but does not have a maximum amount. [If] one makes his entire field peah, it is not peah.
Ask RabbiBookmarkShareCopy