Refer to page 16 of the FLR document
For HSMP extensions: If you have applied for a HSMP extension, you must submit documents that demonstrate the following:
o Your economic activity and your personal earnings to date if you are employed, during your stay under the Programme;
o Evidence of the business you have established, if you are self-employed.
o If you are not economically active you will need to provide evidence to show that you have taken all reasonable steps to do so during your stay in the UK e.g. several job applications relevant to your skills and experience
Details of all the information to send with your extension application can be obtained from the Home Office website:
www.ind.homeoffice.gov.uk
I would include all my bank statements, pay slips, P60 if available, employment contract etc