This is from the IRS website:https://www.irs.gov/individuals/international-taxpayers/frequently-asked-questions-about-international-individual-tax-matters#TopFAQs
The taxation of payments received from Canadian retirement programs that are similar to the U.S. Social Security system receive special tax treatment due to an income tax treaty between the United States and Canadian governments. The way this income is taxed depends on the recipient’s residence.
The special tax treatment applies to payments receive from the following Canadian retirement programs: Canada Pension Plan (CPP), Quebec Pension Plan (QPP), and Old Age Security (OAS)
If the recipient is a resident of the United States, the benefits:
If the recipient is a U.S. citizen or lawful permanent resident (green card holder) who is a resident of Canada, the benefits are taxable only in Canada.
NOTE: Refer to Tax Topic 423, Social Security and Equivalent Railroad Retirement Benefits, for information about determining the taxable amount of your benefits.
For more details, please refer to Publication 519, U.S. Tax Guide for Aliens, Publication 597, Information on the United States-Canada Income Tax Treaty, and Publication 915, Social Security and Equivalent Railroad Retirement Benefits.