Satyam vada! Dharmam cara! – Speak the truth! Practice Virtue! (Taittiriya Upanishad Chapter I, section 11, Verse 1)
Satyameva Jayate nanrtam – Truth alone wins, not untruth (Mundaka Upanishad Chapter Chapter III Verse 6).
Satyam na pramaditavyam – Let there be no neglect of Truth (Taittiriya Upanishad Chapter I, section 11, Verse 1)
By truth is laid out the path leading to the gods (Mundaka Upanishad Chapter III Verse 6).
Truthfulness is excellent. What is excellent is truthfulness only. By truthfulness those who have attained to the state of blissfulness never fall from there. What belongs to sat, namely good people, that is indeed satyam (truthfulness). For this reason seekers of the highest good find delight in truthfulness. (Maha Narayana Upanishad Verse 78 – 1)