The SA105 is the Income from Property pages attached to an SA return. Why would a mortgage provider want that? If they do want it and you used the HMRC software and you can print that bit off. If you used an agent, you can ask the agent to email you the relevant pages.
Could it be an SA302 they want as you are applying for a new mortgage?
The SA105 is the UK property section of a tax return. It is unusual for a mortgage provider to request this.
What is more commonly requested is an SA302, which is the tax calculation you get upon completing the return. If you filed the return yourself online you can view and print this using your online account.
If an accountant did your return, they should be able to do the same.
If you have any further questions, or need us to send you a copy in the post, you can contact us via webchat or call the Self-Assessment helpline on 0300 200 3310 (open 8am - 8pm Monday to Friday, 8am - 4pm Saturday and 9am - 5pm on Sunday).