In the battle of Khandaq, the Muslims dug a ditch around Madina for their defense, so that the enemy could not get across. A man from the enemy side called 'Amr bin 'Abdawud, who was known for his strength, courage and art of fighting managed to get across the ditch. All the Muslims were terrified to fight him and only Imam Ali (A) came forward to fight this man. As the Imam (A) walked towards the enemy, the Prophet (S) said,
'The entire faith (Iman) is advancing to face the entire infidelity (Kufr)."
There was a fierce fight until at last the Imam (A) threw 'Amr down onto the ground and mounted his chest ready to kill him. Just as the Imam (A) was about to kill this enemy of Islam, he spat on the face of our Imam (A). Everybody was certain that because of this insult, 'Amr would meet his death even faster. To their amazement, however, the Imam (A) moved off from 'Amr's chest and walked away. 'Amr attacked the Imam (A) once more and after a short while, the Imam (A) again overpowered and killed him.
After the battle was over, the people asked the Imam (A) the reason why he had spared 'Amr's life when he had first overpowered him. The Imam (A) replied that if he had killed him the first time, it would have not been only for the sake of Allah but also for the satisfaction of his own anger and so he let him free. Then, the Imam (A) controlled his anger and killed 'Amr purely for the sake of Allah.1
Ahadith from Imam Ali
O my God, Master and Lord! can you see yourself tormenting me with your fire after I have professed your unity,
and after the knowledge of you my heart has embraced,
The remembrance of you my tongue has constantly mentioned,
The love of you to which my mind has clung,
And after the sincerity of my confession and my supplication, humble before your lordship?
Far is it from you!
You are too generous to squander him whom you have nurtured...2
1. Subhani, Furugh-e 'Abadiyyat, pg 485-489
2. a selection from the Supplication of Kumail
يا إِلَهِي وَ سَيِّدِي وَ رَبِّي أَ تُرَاكَ مُعَذِّبِي بِنَارِكَ بَعْدَ تَوْحِيدِكَ وَ بَعْدَ مَا انْطَوَى عَلَيْهِ قَلْبِي مِنْ مَعْرِفَتِكَ وَ لَهِجَ بِهِ لِسَانِي مِنْ ذِكْرِكَ وَ اعْتَقَدَهُ ضَمِيرِي مِنْ حُبِّكَ وَ بَعْدَ صِدْقِ اعْتِرَافِي وَ دُعَائِي خَاضِعاً لِرُبُوبِيَّتِكَ، هَيْهَاتَ أَنْتَ أَكْرَمُ مِنْ أَنْ تُضَيِّعَ مَنْ رَبَّيْتَهُ.