It is not required that you provide a citation for information considered 'common knowledge' in your field or discipline. Common knowledge refers to well-established facts or common sense observations within a particular group's understanding. For example, for many people, major historical facts (e.g. 'the World Trade Center towers in New York collapsed on 11th September 2001', 'Canberra is the capital of Australia') are widely considered common knowledge and so would not need to be cited.
In your discipline, there may be common knowledge that your lecturer does not expect you to cite. In Physics it may be referring to Newton's laws of motion, in Business it could be a description of Corporate Social Responsibility, in Music it might be the idea that Mozart was a classical composer. However, if you're writing for a broader audience you may need to provide and cite definitions of such concepts.
When giving specific and/or contested information, it's best to cite it. For example, you'd need to cite claims such as:
- Studies on galaxies have proposed modifications to Newton's laws of motion
- Corporate Social Responsibility adoption is beneficial for the environment
- Mozart's first composition was written while he was living in Salzburg
To determine whether you need to cite the information, ask yourself:
- Is it likely that most people in my audience know this information?
- Does everyone in my audience agree on this information?
- Is the information widely accessible?
If in doubt, cite it.