[Platonov-Rapinchuk][1] is the canonical reference.

**EDIT** A less nuclear option (and one I should have mentioned in the first place is [Morris Newman's Integral Matrices.][2] I think chapter 7 and/or 9 have a(n elementary) proof.


  [1]: http://books.google.com/books/about/Algebraic_Groups_and_Number_Theory.html?id=NpVdWdFq5YMC
  [2]: https://www.dropbox.com/s/w61gpdhx88dq1ur/%5BMorris_Newman%5D_Integral_matrices.pdf