Publication 525 - Taxable and Non-taxable Income
http://www.irs.gov/publications/p525/ar02.html#d0e7312
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Bribes. If you receive a bribe, include it in your income.
http://www.irs.gov/publications/p525/ar02.html#d0e7312
also
Kickbacks. You must include kickbacks, side commissions, push money, or similar payments you receive in your income on Form 1040, line 21, or on Schedule C or Schedule C-EZ (Form 1040) if from your self-employment activity.
