إیّاکُمْ وَالخُصُومَةََ فَإنَّهَا تَشْغَلُ القَلْبَ وَتُورِثُ النِّفاقَ وَتَکْسِبُ الصَّغائِنَ. Keep away from hostility, for it occupies the heart, gives rise to hypocrisy, and creates ill will within the heart(and causes hatred).
Al-Kafi, vol. 2, page 301
Imam Ja'far ibn Muhammad al-Sadiq (peace be upon him)Hadith