Python Selenium accessing HTML source

How can I get the HTML source in a variable using the Selenium module with Python?

I wanted to do something like this:

from selenium import webdriver
browser = webdriver.Firefox()
browser.get(raw_input("Enter URL: "))
if "whatever" in html_source:
    # Do something
else:
    # Do something else

How can I do this? I don't know how to access the HTML source.