O ye who believe! Fear Allah, and (always) say a word directed to the Right:
يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ ٱتَّقُوا۟ ٱللَّهَ وَقُولُوا۟ قَوْلًا سَدِيدًا
Quran 33:70
The same idea in other traditions
Keep thy tongue from evil, And thy lips from speaking guile.
Truth is higher than everything; but higher still is truthful living.
Lying lips are abomination to the Lord: but they that deal truly are his delight.
Him I call indeed a Brâhmana who utters true speech, instructive and free from harshness, so that he offend no one.
By simplicity he will become upright in actions, thoughts, and speech, and he will become veracious; thereby he will truly practise the Law.
The Master said, "Yu, shall I teach you what knowledge is? When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it;—this is knowledge."
The true prevails, not the untrue; by the true the path is laid out, the way of the gods (devayânah), on which the old sages, satisfied in their desires, proceed to where there is that highest place of the True One.