@ebrahimm777 :
I believe Illinois is correct.
Illinois Law 35 ILCS 5, Section 304 (a)(2)(B)(1) states:
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Compensation is paid in this State if:
(i) The individual's service is performed
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entirely within this State;
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https://www.ilga.gov/legislation/ilcs/ilcs5.asp?ActID=577&ChapterID=8
Therefore, if your bonus represented compensation for services performed entirely in Illinois, it is taxable by Illinois. It may also be taxable by your state of residence at the time you received it - in which case you could normally claim a credit for the taxes paid to Illinois on the income taxed by both.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
