I heard that Yudhishthira was great dharmatma and he never lied. And because of his this level of dharma prayanta, his chariot always stay above ground, it never touches the ground whenever Yudhishthira sit on a chariot. Is it really true that Yudhishthira's interest toward dharma makes him this level of dharmatma? Anywhere mentioned about this fact?
Yudhishthira never talks untruth. Hence his chariot didn't touch the ground. But after he told the untruth because of the reason that the untruth in that particular context(stopping Drona's fight in the war) is sinless and better than the truth, his chariot along with animals touched the ground.
It is mentioned in the following passage of SECTION CXCI of Drona Parva of Mahabharata
"Meanwhile, O monarch, Govinda, knowing that Drona, that foremost of warriors, was capable of sweeping all the Pandavas off the face of the earth, became much distressed. Addressing Yudhishthira he said, 'If Drona fighteth, filled with rage, for even half-a-day, I tell thee truly, thy army will then be annihilated. Save us, then, from Drona. under such circumstances, falsehood is better than truth. By telling an untruth for saving a life, one is not touched by sin. There is no sin in untruth spoken unto women, or in marriages, or for saving king, or for rescuing a Brahmana.' 1 While Govinda and Yudhishthira were thus talking with each other, Bhimasena (addressing the king) said, 'As soon, O monarch, as I heard of the means by which the high-souled Drona might be slain, putting forth my prowess in battle, I immediately slew a mighty elephant, like unto the elephant of Sakra himself, belonging to Indravarman, the chief of the Malavas, who was standing within thy army. I then went to Drona and told him, 'Aswatthaman has been slain, O Brahmana! Cease, then, to fight. Verily, O bull among men, the preceptor did not believe in the truth of words. Desirous of victory as thou art, accept the advice of Govinda. Tell Drona, O King, that the son of Saradwat's daughter is no more. Told by thee, that bull among Brahmanas will never fight. Thou, O ruler of men, art reputed to be truthful in the three worlds.' Hearing those words of Bhima and induced by the counsels of Krishna, and owing also to the inevitability of destiny, O monarch, Yudhishthira made up his mind to say what he desired. Fearing to utter an untruth, but earnestly desirous of victory, Yudhishthira distinctly said that Aswatthaman was dead, adding indistinctly the world elephant (after the name), Before this, Yudhishthira's car had stayed at a height of four fingers' breadth from the surface of the earth; after, however, he had said that untruth, his (vehicle and) animals touched the earth. Hearing those words from Yudhishthira, the mighty car-warrior Drona, afflicted with grief, for the (supposed) death of his son, yielded to the influence of despair. By the words, again, of the Rishis, he regarded himself a great offender against the high-souled Pandavas. Hearing now about the death of his son, he became perfectly cheerless and filled with anxiety; upon beholding Dhrishtadyumna, O king, that chastiser of foes could not fight as before.'"