In the Holy Qur'an

And those who restrain (their) anger and pardon men; and Allah loves the virtuous (3:134)

In the Traditions

The Prophet said, "Shall I tell you who is the toughest and strongest from among you?" They replied, "Yes Messenger of Allah, do tell us",
so the Prophet said "the strongest and toughest of you is he who, when he is happy, his happiness does not lead him to committing a sin or anything wrong, and when he gets angry, his anger does not prevent him from speaking the truth, and when he is empowered in any way, he does not take hold of that which is not lawfully his."

Imam Ali said, "There are six things by which people's intellects may be tested: clemency at the time of anger, patience at the time of fear, calculation when faced with desire, God wariness at all times, amicableness, and minimal engagement in disputation."