Most mortgage programs require that the borrower provide documentation to prove that the income which they stated on the loan application is accurate.

The required income documents vary depending on which mortgage program you decide will benefit your financial situation.

In addition, you will most likely be asked to sign an IRS form 4506 or 4506t. These allow the lender to get copies of your tax return or a transcript of your tax return to verify what you have reported to the IRS. They will usually not exercise this option unless you are late on your payment in the first month or two.
However, if they do access your tax returns and find them to be different than what you provided to them, they can demand immediate repayment of your mortgage. Further, they may attempt to have you charged with mortgage fraud.

The most common income documents that are requested for a mortgage loan are your last two years worth of W2's and/or tax returns, 1 full month worth of most recent pay-stubs, and a verification of employment.

If you don't have a standard W2 or 1009 income, the lenders offer various forms of proof, if any, for proof of income...If you have a tricky income to document or use creative accounting to minimize your tax exposure, your bank statements alone can help you qualify for a loan...6-12 months is the norm but 3 may work, lender pending...