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How do I report this cash bonus/tip on income tax return?
Daniel covered the correct way to file on the returns, I'm chiming in specifically to discuss the question of whether it could be a gift. The IRS will classify it as a tip even if the person giving it says it's a gift if a service was rendered before the gift was given. The only way that you could make a case to the IRS that it was a gift is if you have a personal relationship outside of the working environment, and the person giving the gift provides an explanation for the motivation behind the gift.  Such explanations as "Happy Birthday" or "Congratulations on graduating" or other special occasions could be gifts. But "you did a good job, and I just want to reward you for your effort" is not a reason someone gives a gift, and the IRS will penalize you if you do not have evidence that it was a gift rather than a tip.