Search a Git repository by commit message or Search the actual content of commits through a repo's history

How to search a Git repository by commit message?

Source: https://stackoverflow.com/questions/7124914/how-to-search-a-git-repository-by-commit-message

Someone from my team or I checked some source code into Git with the commit message “Build 0051”.

However, I can’t seem to find that source code any more - how do I extract this source from the Git repository, using the command line?