That was from taitreya Upanishad.. translation taken from internet only..
vedamanuchyacharyontevasinamanushasti .
satyam vada . dharmam chara . svadhyayanma pramadah .
acharyaya priyam dhanamahritya prajatantum ma vyavachchetsih .
satyanna pramaditavyam.h . dharmanna pramaditavyam.h .
kushalanna pramaditavyam.h . bhutyai na pramaditavyam.h .
svadhyayapravachanabhyam na pramaditavyam.h ..
Having taught the Vedas, the teacher thus instructs the pupil: Speak the truth. Practise
dharma. Do not neglect the study of the Vedas. Having brought to the teacher the gift, desired by him, enter the householder’s life and see that the line of progeny is not cut off.
Do not swerve from the truth. Do not swerve from dharma. Do not neglect personal
welfare. Do not neglect prosperity. Do not neglect the study and teaching of the Vedas.
Adiyen