From The United States Constitution, Article I, section 5:
'Each House shall keep a journal of its proceedings, and from time to time publish the same, excepting such parts as may in their judgment require secrecy; and the yeas and nays of the members of either House on any question shall, at the desire of one fifth of those present, be entered on the journal.'
More information about the journals can be found in the Library of Congress online guide.