1. Reports > Financial > Balance Sheet > Customize.
2. On the Edit Layout section
3. Expand for Equity > then choose reference row for Net Income.
4. On the Report field > choose your custom Cash Basis Income Statement report.
5. Click Save.
Note: For information on how to customize Income statement to Cash basis reporting refer to the article: Setting up Cash Basis Reporting
Note that this will result to the Balance sheet not being balanced since Cash Basis Balance sheet is currently an enhancement.