Hast thou really slain today that sinful wight who was exceedingly dear to Suyodhana, and who, intoxicated with pride of heroism, used always to brag in the assembly of the Kurus? Encountered in battle, doth that wretch lie today on the field, his limbs exceedingly mangled with sky-ranging shafts sped by thee from thy bow and all steeped in blood? Have the two arms of Dhritarashtra's son been (at last) broken? Have those words been unfulfilled, uttered from folly by him who, filled with pride, used to always boast in the midst of the kings for gladdening Duryodhana, saying, 'I will slay Phalguna'? O son of Indra, hath that Karna of little understanding been slain by thee today, that Suta's son who made the vow that he would not wash his feet as long as Partha lived?
http://www.sacred-texts.com/hin/m08/m08066.htm
Whilst I was tortured in the midst of the assembly by the wicked-souled sons of Dhritarashtra, my wrath, suddenly provoked, became cooled at sight of Karna. Even while listening to the harsh and bitter words of Karna himself on that occasion of our match at dice, to the words, that Karna uttered from desire of doing what was agreeable to Duryodhana, my wrath became cooled at sight of Karna's feet. It seemed to me that Karna's feet resembled the feet of our mother Kunti. Desirous of finding out the reason of that resemblance between him and our mother, I reflected for a long time. With even my best exertions I failed to find the cause.
http://www.sacred-texts.com/hin/m12/m12a001.htm
https://www.speakingtree.in/allslides/why-did-arjuna-wash-karnas-feet
http://www.sacred-texts.com/hin/m08/m08066.htm
Whilst I was tortured in the midst of the assembly by the wicked-souled sons of Dhritarashtra, my wrath, suddenly provoked, became cooled at sight of Karna. Even while listening to the harsh and bitter words of Karna himself on that occasion of our match at dice, to the words, that Karna uttered from desire of doing what was agreeable to Duryodhana, my wrath became cooled at sight of Karna's feet. It seemed to me that Karna's feet resembled the feet of our mother Kunti. Desirous of finding out the reason of that resemblance between him and our mother, I reflected for a long time. With even my best exertions I failed to find the cause.
http://www.sacred-texts.com/hin/m12/m12a001.htm
https://www.speakingtree.in/allslides/why-did-arjuna-wash-karnas-feet
