Al-Imam al-Bukhari narrated in his Sahih from Hadith Abu Hurairah that the Prophet said, "Man lam yada' qawluzuri wal'amali bihi falaysa lillahi hajatun an yada' ta'amahu wa sharabahu." Whoever does not leave off lying.
some of the scholars even believe, like Ibrahim an-Nakha'i and others from the Salaf, they actually believed that the person's fasting will be null and void.