Satyam bruyat priyam bruyat, na bruyat satyam apriyam. Priyam cha nanritam bruyat, esha dharmah sanatanah. Speak the truth. Speak what is pleasant. Do not speak an unpleasant truth. Do not speak a pleasant untruth. This is the eternal law.
The verse is from the Manusmriti, the ancient law book, chapter 4, verse 138.