The Schleitheim Confesion, Article 1

,Since direct translations from the original German are very long-winded, I've decided to use the summaries from Wikipedia instead.

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BAPTISM 

Baptism is administered to those who have consciously repented and amended their lives and who believe that Christ died for their sins and who request it for themselves. Infants, therefore, were not to be baptized.

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Baptism was the most visible issue that separated the early Anabaptists from both the Church of Rome and the Reformers. In fact, the very name "Anabaptist" is a reference to their practice of "baptizing again", giving baptism to those who had been through the practice of infant baptism.