tell a (little) white lie
To tell a seemingly small, insignificant, or harmless lie, often presumably in order not to offend or upset someone. I knew Jenny would be upset if she knew I didn't want go to the movies with her tomorrow, so I just told her a white lie about having to take care of my elderly grandmother. Don't get into the habit of telling little white lies, or, pretty soon, you'll start telling big, fat, ugly ones.