Mbh 5.8.4a

Vaisampayana continued, “Then, O descendant of Bharata, the king spoke of his meeting with Duryodhana and gave a detailed account regarding that promise of his and that boon granted by himself. And Yudhishthira said, ‘O valiant king, it has been well-done by thee that being pleased at heart thou hast plighted thy truth to Duryodhana. But good betide thee, O ruler of the earth, I ask thee to do one thing only. O king, O best of men, thou wilt have to do it solely for my sake, though it may not be proper to be done. O valiant one, hear what I submit to thee. O great king, thou art equal to Krishna on the field of battle. When, O best of kings, the single combat between Karna and Arjuna will take place, I have no doubt thou wilt have to drive Karna’s car. On that occasion, if thou art inclined to do good to me, thou must protect Arjuna. O king, thou must likewise so act that the Suta’s son Karna may be dispirited and the victory may be ours. Improper it no doubt is; but, O my uncle, for all that thou must do it.’ Salya said, ‘Good betide thee. Listen, O son of Pandu. Thou tellest me to so act that the vile son of the Suta may be dispirited in fight. To be sure, I shall be his charioteer on the field, for he always considers me equal to Krishna. O tiger like descendant of Kuru, I shall certainly speak to him, when desirous of fighting on the field of battle, words contradictory and fraught with harm to him, so that bereft of pride and valour, he may be easily slain by his antagonist. This I tell thee truly. Asked by thee to do it, this I am determined to do, O my son. Whatever else I may be able to bring about, I shall do for thy good.