In the current market every seller is anxious to know that the buyer will go through with the purchase. While a cash sale is greatly preferable to an offer that comes with a financing contingency, it can give additional reassurance to the seller to have documentary evidence that the buyer has the necessary accessible funds to close on the deal.
In the case of a distressed sale (a foreclosure or a short sale), the lender will most likely make it a condition of any cash purchase offer that it must be accompanied by proof of funds documentation. Even regular sales can make this a stipulation. It is a good idea to provide it whether or not it is asked for, because it provides extra strength to your offer.
If you have adequate funds to close on the purchase already sitting in a bank account, the simplest form of proof of funds documentation is a copy of a recent bank statement for that account. You can blank out any details on the statement that are unrelated to the issue in hand, but the seller will want to see contact information for the bank holding the funds, the buyer's name, and the statement date.
If the funds are in more than one bank account, such as a checking account and a savings account, or separate bank accounts for joint purchasers, this isn't a problem. Just send copies of however many accounts are needed to make up the total. You have no requirement to use the funds from these particular accounts when you close on the purchase. The funds can come from anywhere that is convenient to you.
Because of fluctuating currency exchange rates the seller may require that your proof of purchase is specified in US dollars, or accompanied by a bank letter converting the funds into US dollar equivalent.
If you are getting the funds to purchase your new home from a refinancing or other loan, you need to be able to show that the loan has been approved. In this situation you will need to provide a letter from your bank to indicate that you have line of credit available to cover the purchase price. If the loan hasn't been approved you can't claim to be a cash purchaser.