What other seminal document does King refer to in this paragraph? Which of King’s three reasons for being in Birmingham is supported by this distinction between just and unjust laws?

You express a great deal of anxiety over our willingness to
break laws. This is certainly a legitimate concern. Since we so
diligently urge people to obey the Supreme Court’s decision of 1954
outlawing segregation in the public schools,[6] at first glance it may
seem rather paradoxical for us consciously to break laws. One may
well ask: “How can you advocate breaking some laws and obeying
others?” The answer lies in the fact that there are two types of laws:
just and unjust. I would be the first to advocate obeying just laws.
One has not only a legal but a moral responsibility to obey just
laws. Conversely, one has a moral responsibility to disobey unjust
laws. I would agree with St. Augustine[7] that “an unjust law is no law
at all.”