Ali had defeated an enemy knight, but just as he was about to strike the death-blow the knight spat in Ali's face.
Ali immediately threw his sword away and said to the knight, "Arise!"
"I don't understand," said the knight as he got to his feet.
"I wield that sword for God," said Ali. "Let it not be said that I slew someone from anger. You spat upon me, and for that I forgive you. God's mercy has made me your friend, and I open my heart's door to you."
When he heard Ali's words, the knight converted to Islam.
Inspired by: The English prose version of Rumi in Tales from the Masnavi by A. J. Arberry.
Notes: This is story 19 in the book. Compare the story of Hormuzan's Conversion, who did not want to be converted under fear of the sword. You can read more about Ali, Mohammad's son-in-law, at Wikipedia. The knight in the story converts from the "Magian" religion, i.e. Zoroastrianism.