git blame on the file. It'll show you the commit ID,
the date and time, and who committed it- for each line. Then just copy out
the commit identifier and you can use it in
git show <commit>.
For example, I've got a file, called test.txt, with lines added on
$ cat test.txt
$ git blame test.txt
^410c3dd (Leigh 2013-11-09 12:00:00 1) First line.
2365eb7d (Leigh 2013-11-09 12:00:10 2) Second line.
The first bit is the commit ID, then name, then date, time, time zone,
and finally the line number and line contents.