Here’s what I found doing a simple Google search:
“Every cent of earned income through side jobs, gambling, prize money, and so forth must be documented. You may use your bank statements or check stubs to document the income. The IRS does not request this documentation when you file your taxes at the beginning of the year, but you need to keep it in case you are audited.”
So nothing needs to be attached but you need to keep the proof of the income via check stubs and/or bank statements as stated above.