The paper version of I-94 says the following:
Retain this permit in your possession; you must surrender it when you leave the U.S. Failure to do so may delay your entry into the U.S. in the future.
However in practice, your exit is always recorded when you enter Canada overland as the two countries share immigration data. Therefore presuming that your exit was properly recorded, is there also a legal obligation to return the physical I-94 paper to Canadian immigration?
Please cite the relevant sections of US immigration law, if possible.