It is narrated from Sayyiduna Adi bin Hatim (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The most blessed and the most ill-omened thing of a person is what lies between his two jaws (that is, his tongue)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14270