Truthfulness is a very important and core value in Islam
In the Quran Allah says:
“O you who believe! Fear God, and be with those who are true (in word and deeds).” (Quran 9:119)
A companion asked the Prophet Muhammad (peace be upon him): “Can a believer be a coward? He said, “Yes, he can be coward. Then it was asked whether a believer could be a miser? He replied, “Yes, he can be a miser”, it was then asked whether a believer could be a liar? He replied, “No, a believer cannot be a liar.” (Imam Malik, Mishkaat)
The Prophet (SAW) said, "The signs of a hypocrite are three: Whenever he speaks he tells a lie; whenever he is entrusted he proves dishonest; whenever he promises he breaks his promise." (Sahih Bukhari)
The Prophet said, "Truthfulness leads to righteousness, and righteousness leads to Paradise. And a man keeps on telling the truth until he becomes a truthful person. Falsehood leads to Al-Fajur (i.e. wickedness, evil-doing), and Al-Fajur (wickedness) leads to the (Hell) Fire, and a man may keep on telling lies till he is written before Allah, a liar." (Sahih Bukhari)
Allah's Apostle (the Prophet Muhammad) said thrice, "Shall I not inform you of the biggest of the great sins?" We said, "Yes, O Allah's Apostle" He said, "To join partners in worship with Allah: to be undutiful to one's parents." The Prophet sat up after he had been reclining and added, "And I warn you against giving forged statement and a false witness; I warn you against giving a forged statement and a false witness." The Prophet kept on saying that warning till we thought that he would not stop. (Sahih Bukhari)