Hausdorff property of Manifolds

Is the fact that all manifolds are hausdorff spaces a part of the definition, or can this be proven from the fact that it is a set which is locally isomorphic to open subsets of a hausdorff space?

P.S. if it can be proven I dont want to know the proof, I want to keep working on it, I just want to know that hausdorff property is not some assertion put in from the outset.