Step 1: Add the submodule
git submodule add git://some_repository.git some_repository
Step 2: Fix the submodule to a particular commit
By default the new submodule will be tracking HEAD of the master branch, but it will NOT be updated as you update your primary repository. In order to change the submodule to track a particular commit or different branch, change directory to the submodule folder and switch branches just like you would in a normal repository.
git checkout -b some_branch origin/some_branch
Now the submodule is fixed on the development branch instead of HEAD of master.
From Two Guys Arguing — Tie Git Submodules to a Particular Commit or Branch .