CRA will know the amount withdrawn from your RRSPs under the Home Buyer's Plan by the issuance of a T4RSP slip.  If you make a withdrawal from your RRSP for the HBP or LifeLong Learning Program (LLP) in the calendar year, you will be issued a T4RSP with an amount in Box 27 showing these withdrawals.  Withdrawals in 2017 would be reported on the 2017 income tax return from the slip.

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/rrsps-related-plans/what-hom...