Asalamu alaykum,
Imam Ali (عليه السلام) was fighting the most fiercest known warrior in Arabia, having known as a warrior being equal to 100 men. In the heat of the battle, the fierce warrior spat in Imam Ali's face, just as Imam Ali (عليه السلام) was going for the final blow. Imam Ali (عليه السلام) then stepped back, and spent a few seconds or possibly a minute, walking around and then came back and finished the warrior off.
People later on asked him why he stepped back and stopped fighting for a few moments. He replies that he wanted to do the kill sincerely for the sake of Allah (سُبْحَانَهُ وَ تَعَالَى), and not for himself at all.