It is narrated that one day the Prophet of Islam (PBUH) asked his companions:

Among the signs of the belief, which one is the strongest? Each of the companions said something; such as Salaat, Zakat, fasting, Hajj, Jihad, etc. The Prophet (PBUH) said: All of these which you mentioned are great and virtuous, but none of them is the reply to my question; rather the strongest sign of the belief is “to like for the sake of God and to dislike for the sake of God [1].” In another Hadith (narration), the Prophet (PBUH) said: Every time Jibril/Gabriel (AS) came to me, advised me, and the last of them was: Avoid dislike of the people, because enmity reveals the hidden fault, and destroys the human being’s dignity [2].

[1] Daastaan-e-Raastaan, Martyr Mortaza Motahhari (RA), Page 227.

[2] Osul Kafi, Vol. 3, Page 411.

Source: en.shafaqna.com