This is quite common. I assume you are purchasing a home and the bank statement has your moms name on it. Even though she is not cosigning for the mortgage, the bank needs to know that you are allowed access to the funds at all times. Its a silly condition, but an easy one to meet.
Hmmm...sounds a little odd? Are you getting a gift from your mother for the down payment as allowed on a FHA loan? If so, you'll need to have your mother complete a gift letter as required by the FHA lender. If you are getting a gift on a conventional loan, she'll need to complete a gift letter as well as prove with her bank account that she had the funds to gift to you....(fha does not require proof of funds). Not sure if this is what you're asking, but if not, please post another question.