Is it necessary to show that the 800,000 THB for a retirement visa in Thailand came from overseas?

For the 800'000THB you need either proof it came from abroad or you earned and paid tax for it here in Thailand.

Or on the other hand, IF you are on an entry non B visa for 90 days, you cannot apply for a retirement extension directly, You have to leave Thailand and apply at a Thai Embassy/Consulate for the Non Immirgant O visa based on being over 50 years of age, get 90 days on entry and apply after 45 days for the 1 year Extension of Stay, but be aware the 800.000 Thb needs to be at least 2 months prior application date in a bank account on your name.

Check your current stamp. You most likely are on an "extension of stay" stamp. If this is right, what you are going to do is, cancel your extension of stay based on employment and immediately directly apply for an extension based on retirement using your banked 800K baht in your Thai bank account sitting there for more than two months as of application day. In this case, 800k baht doesn't have to be internationally transferred.

On the other hand, IF you are on an entry non-B visa for 90 days, you cannot apply for a retirement extension directly. You have to leave Thailand, enter Thailand again, and then apply for an in-country initial NOn-O visa based on retirement first. In this case, the funds of 800 baht in your Thai bank account have to be shown as coming from abroad.