In which sources did the SOLID principles first appear?

The SOLID principles were first formulated by Robert C. Martin, also known as "Uncle Bob." They appeared in the early 2000s and were described in his articles and books, such as "Agile Software Development, Principles, Patterns, and Practices" (2002).

SOLID is an acronym for five fundamental principles of object-oriented design:

• S — Single Responsibility Principle
• O — Open/Closed Principle
• L — Liskov Substitution Principle
• I — Interface Segregation Principle
• D — Dependency Inversion Principle

These principles have gained widespread adoption in the developer community and are used to improve the quality and maintainability of code.