Yes. In 99% of cases it is illegal. There are some cases when it ok to keep it (like if one pays off their loan too early [1]) but you will be criminally prosecuted if you do not return money you have no reason to believe it should be yours [2].

[1] https://www.cnn.com/2021/02/16/business/citibank-revlon-laws...

[2] https://www.ktvu.com/news/woman-jailed-after-refusing-to-ret...