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.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.
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.
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.
THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation.