you have to show the money was lawfully obtained----so a letter confirming it as a gift should suffice. its then up to the case officer to say if he/she wants further evidence.
7.5. Cash savings – specified evidence 7.5.1. The evidence required for cash savings is specified in Appendix FM-SE: 11. In respect of cash savings the following must be provided: (a) Personal bank statements showing that at least the level of cash savings relied upon in the application has been held in an account(s) in the names of the person and their partner jointly throughout the period of 6 months prior to the date of application. (b) A declaration by the account holder(s) of the source(s) of the cash savings.