Exact requirements for Cash Savings declaration?

My wife is applying for a settlement visa from the UK
We have sufficient cash savings for over six months.
From Immigration Directorate Instruction Family Migration Appendix FM I know that:

11. In respect of cash savings the following must be provided:
(b) A declaration by the account holder(s) of the source(s) of the cash savings.

We cannot find any information on exactly what form this declaration must take.
Can anyone advise us?

We have been advised that 'A solicitor or attorney should draft a statutory declaration whereby you
declare the source of your savings in the bank account.' but here in the USA no one understands what this means.

Can we not write ' The cash savings in our bank accounts were derived from a savings from regular income over the last ten years.'
And then have our declaration notarized (to prove our identities were verified and signatures witnessed).
Thanks