Confucius. Book 1, Chapter 4.
The philosopher Tsang said, "I daily examine myself on three points: ‹ whether, in transacting business for others, I may have been not faithful; ‹ whether, in intercourse with friends, I may have been not sincere; ‹ whether I may have not mastered and practiced the instructions of my teacher."
Text source: Lun Yu, The Confucian Analects. Translated by James Legge.