Settling "What is source of funds document?" means knowing which question is being asked. Money goes in with almost no questions and comes out with several, and that asymmetry catches most people at their first withdrawal. A casino is obliged to know who it is paying, where that person lives and where the money being staked came from, and it usually asks at the moment it is about to pay rather than when the account opened. Knowing which three checks exist, and what satisfies each, turns a stalled payout into a short errand.
An answer that will not come has consequences worth knowing in advance. An account that cannot satisfy a source of funds request is usually frozen rather than closed, with the balance held until it can, and the deposits already made are normally returnable to their origin. That is the reason to deposit from an account in the same name from the start: every check further down the line reads more easily for a paper trail that began in the right place.
The identity check is the simplest. A government document with a photograph, a full name and a date of birth settles it: a passport, a driving licence or a national identity card. The name on it has to match the name on the account exactly, which is where most rejections come from — a shortened first name, a maiden name, a middle name present in one place and absent in the other. Photographs of a document are read by a person or a machine, so the corners have to be in frame and the text legible rather than washed out by a flash.
Source of funds asks for paper, not an explanation
Proof of address dated within three months
Salary, pension and a house sale as ordinary answers
When each check arrives, and why the third comes late
A frozen account, and what it means for the balance