U.S. social security benefits paid to a resident of Canada are taxed in Canada as if they were benefits under the Canada Pension Plan, except that 15% of the amount of the benefit is exempt from Canadian tax. IRS Pub 597 As a result of the US-Canada treaty, as long as you're reporting the income on your Canadian tax return, you don't have to report them on a US tax return.
Here is a TurboTax article that explains how to file your US social security benefits in Canada.
Here is another helpful TurboTax article with tax tips for filing overseas.
