Per IRS Publication 17 Your Federal Income Tax (For Individuals), on page 72:
Child support payments. You shouldn’t report these payments on your return. See Pub. 504 for more information.
Per IRS Publication 504 Divorced or Separated Individuals, on page 14:
Child support. To determine whether a payment is child support, see the discussion under Certain Rules for Instruments Executed After 1984, later. If your divorce or separation agreement was executed before 1985, see the 2004 revision of Pub. 504 available at IRS.gov/FormsPubs.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.