Where/How do I report the return date for B-2 Visa?

We were given B-2 visa on Dec. 6/2008 upon entry to the USA. We were told that it is required by law that we report when we return to Canada by the US immigration officer. We forgot to ask becuase we were late for the flight.
We plan to to return on May 27th by air. We are retired Canadian and have been snow birding in the USA for 4 to 6 months a year for the last 3 years but this is the first time we were givin B-2 visa.
Could someone let us know where to report our entry back to Canada? Will Canadian Immigration officer prove our return in 6 months? Do we get a stamp on the passport as a proof?
Thank you in advance,

Were you given an I-94 document stapled in your passport? If so, then you just hand that over to Canadian officials when you return, and they forward it to the appropriate US office.

No. We were not given any documents. We just got a stamp on the passport which says "B-2 for Six months".
Where can we get the I-94 document?
Thanks,

You don't need anything, just return as per usual. If you told them you were entering for several months they probably just stamped it to fix a date for your departure, in case you overstay and the next CBP guy who looks at your passport is aware of what your departure date should have been.

You may want to request an I-94 next time you enter, costs $6, then they have specific information as to when you left next time you leave and it basically just saves an argument with CBP. Plus if you're in the US for long periods you will find banks etc. may ask to see your I-94 as proof of ID so for $6 it's worth having one.

There's no way of reporting your re-entry into Canada, CBSA is supposed to make a note but they usually don't, CBP has access to that database.