You should absolutely report since you have an obligation to declare your worldwide income every year in the US. However, if this is your only worldwide income, then you probably are not obligated to file a return since it is below the threshold amount.
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As a United States Citizen you are required to report your income on a world wide basis. If you have a filing requirement in the United States (you would need additional income from other sources), then you will be required to report this income. Check with a local tax professional to determine if your income from all sources would require you to file a return and report this income.
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The answer is "yes," you have to report this income if your total income is over the threshold for filing a return. But I suspect your real question is whether you have to pay tax on the income, and that depends on which country the payment is coming from, exactly what it is for, and other factors in your overall income picture, even though you are retired.
You should consult with a tax professional about the details.