Could not find gem 'sass (>= 0)' in any of the gem sources listed in your Gemfile

Ruby version - ruby 1.9.2p180 (2011-02-18) [i386-mingw32]
I ran the command 'gem install rails --pre' to install 'Rails 3.1.0.beta1'.

When checking 'rails -v', it gives following errors:
Could not find gem 'sass (>= 0)' in any of the gem sources listed in your Gemfile

Steps to fix the error:
Fixed the error by running the command 'gem install sass' separately.

And then again I checked rails version, it gave:
Rails 3.1.0.beta1