Abu Dawud Book 30, Hadith Number 4251.
Chapter : Restraining the tongue during the period of commotion.
Narated By Abu Hurairah : The Prophet (PBUH) said: There will be civil strife (fitnah) which will render people deaf, dumb and blind regarding what is right. Those who contemplate it will be drawn by it, and giving rein to the tongue during it will be like smiting with the sword.