Michael F. Atiyah, [Convexity and commuting Hamiltonians][1] (1982), Theorem 1.

Dusa McDuff and Dietmar Salamon, [Introduction to symplectic topology][2] (2nd ed., 1998), Lemmas 5.51 and 5.54.

Michèle Audin, [Torus actions on symplectic manifolds][3] (2nd ed., 2004), Corollary IV.3.2.


  [1]: http://www.ams.org/mathscinet-getitem?mr=642416
  [2]: http://www.ams.org/mathscinet-getitem?mr=1698616
  [3]: http://www.ams.org/mathscinet-getitem?mr=2091310