Ala. Code § 11-81-171

The governing body of the borrower authorizing bonds under the provisions of this article may make provision for any of such bonds to be called for payment on any interest payment date before maturity, with or without a premium, upon such terms and conditions as the governing body may prescribe; provided, however, that the amount of premium on any bonds shall not cause the yield to be more than six percent per annum from the date of the sale of such bonds to the date of their redemption.