The Holy Prophet (S) said: "Affliction caused by the tongue is worse than (that caused by) the strike of the blade of a sword."
Bihar-ul-Anwar, vol. 71, p. 286
The Holy Prophet (S) said: "Among all things, the tongue deserves to be prisoned longer than anything else." (Because most of our sins are committed by it, such as backbiting, telling lies, defaming, mocking, insulting, etc.)
Bihar-ul-Anwar vol. 71, p. 277
Imam Amir-ul-Mu'mineen ‘Ali (‘a) said: "Do contemplate before speaking so that you may guard yourself from (committing) errs."
Ghurar-ul-Hikam, p. 228
The Holy Prophet (S) said: "The destruction of man lies in three (things): his stomach, his lusts, and his tongue."
Waqayi'-ul-'Ayyam, p. 297
Imam al-Baqir, the fifth Imam, (‘a) said: "No one is safe from sins unless the one controls his/her tongue."
Bihar-ul-Anwar, vol. 78, p. 178