Ann,
The documents needed all relate to your husband meeting the UKVI Financial requirement.

If your husband is self-employed then he will need to apply under Category F or Category G

Under Category F he'll need to prove he is in self-employment at the date of application and prove his income from the last full financial year to meet the appropriate financial requirement.

Under Category G he'll need to prove he is in self-employment at the date of application and choose to use an average of the income received in the last 2 full financial years to meet the financial requirement.

For self-employed Sole Trader or Partner he'll need to submit official documents (Tax Returns)that clearly show taxable income at or above the threshold.
Means business revenue less business expenses.
If the accounts/tax returns show he is being taxed on an amount less than the appropriate Financial Requirement income threshold then the application will fall for refusal.

As a help for Sole Traders / Partners he should secure the following:-

Amount of tax payable, paid and unpaid for the financial year(s) being relied on.
- self-assessment tax return
- statement of Account (SA300 or SA302).
- Proof of registration with HMRC as self-employed (Unique Tax Reference Number (UTR).
- If held, then business bank statements for the same 12-month period as the tax return(s).
- Personal bank statements for the same 12-month period as the tax return(s) showing that the income from self-employment has been paid into an account in the name of the person or in the name of the person and their partner jointly.

- Documentary evidence of ongoing self-employment through evidence of payment of Class 2 National Insurance contributions.

If his business is required to produce audited accounts then he should submit.
If his business is not required to produce audited accounts, then he can submit unaudited accounts together with an accountant certificate of confirmation. The accountant will need to be a member of a UK Recognised Supervisory Body

Although he cannot include savings as part of his financial submission he will be able to include non-employment income (such as property rental)
He must review section 6 of the UKVI Financial requirement for these Category C Non-employment income – requirements.
Look specifically to section 6.1 for guidance on this and documents to submit.

Cash Savings are covered under Category D (Section 7.1 of the Financial Requirement)
But do be aware that Cash savings [B]cannot be combined with self-employment income, or with income from employment as a director of a limited company[/UK]

Be careful about being a director of a limited company who receives dividend income.
This may be counted as income as non-employment income under Category C, unless the company is of a type specified in paragraph 9(a) of Appendix FM-SE (i.e. it is in sole or limited family ownership).

The full details relating to Self-employment or Director of a limited company can be found under Section 9 of the Financial Requirement

For details specifically related to the documents needed to be submitted as a Director then look under Section 9.8. (Director of a specified limited company in the UK – specified evidence) of the Financial Requirement.

The UK Financial Requirement - Appendix FM Section 1.7 can be found by clicking on this link

Your husband absolutely must give priority to understanding the requirement and the specific documents required.

Hope this will help you to start collecting the documentary evidence.