When applying for a personal loan the lender will need some documents and information from you besides just proof of income.
The lender will want to know your credit score and if the income you make is from a job they'll want to verify that you do have the job you say you do and know how long you've been at that job for.
The form of proof of income from your employer that should work for the personal loan are pay stubs which also known as a Pay Slip or Paycheck Stub which shows how much income you make.
If you're on social security and need a personal loan you can go to the social securities website and in the left column of the top services open the services for people currently receiving benefits section and request a proof of income letter and then they'll send you a letter verifying your social security benefit information.