During the battle of Siffin, an arrow went into the leg of Amir al-Mo'minin 'Ali ibn Abi Talib (peace be upon him). The Imam was in great pain because of this injury, and blood was flowing out of his wound. However much the Muslims tried, they were not able to remove the arrow.
The Muslims went to Imam Hasan al-Mujtaba (peace be upon him) and asked him what would be the best way to remove the arrow from his father's leg? Imam Hasan replicd, "Be patient and wait until my father stands for Salat, then at that time, pull the arrow out of his leg because at the time of Saldt, my father is so deeply engrossed in communicating with his Lord, asking and begging Him for his needs, that he will not feel the pain."
The people acted according to what Imam Hasan (peace be upon him) told them, and while The Commander of the Faithful 'Ali (peace be upon him) was praying to his Lord, they managed to pull the arrow out. After Imam 'Ali finished his Salat, he noticed that blood was coming out of his leg and asked those around him as to what had happened. The people replied that while he was praying, they removed the arrow from his leg!
source : http://www.maaref-foundation.com
