Sahih Al Bukhari Hadith Number 1294

Narrated `Abdullah: the Prophet (peace be upon him) said, “He who slaps his cheeks, tears his clothes and follows the ways and traditions of the Days of Ignorance is not one of us.”

23  Funerals (Al-Janaa’iz)
Hadith number 1294
Volume no 2
Chapter: 35 (He who tears off his clothes (when afflicted with a calamity) is not from us)