It was not mentioned whether you earned that foreign income as self-employed or as an employee. 

As per Canada Revenue Agency (CRA): you would fill out the form T2125 Statement of Business activities or professional activities if you're self-employed. US business income is considered foreign business income thus, you have to report your foreign business income into Canadian dollars in form T2125. 

On the other hand, line 10400 is used when foreign employment income is earned outside Canada from a foreign employer not if you're self-employed.

I also strongly recommend you to check this TurboTax blog: United States-Canada Tax Treaty to Reduce Double Taxation for more information.

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