The confrontation between Duryodhana and Krishna happens in the Bhagwata Yana Parva of the Udyoga Parva of Mahabharata. The links to the entire incident are as follows:
Krishna decides to solicit for peace as an emissary of the Pandavas in the Kuru court.
Thus addressed, Janardana replied unto Yudhishthira the just, saying, 'I will go to the court of the Kurus for the sake of both of You. If without sacrificing your interests I can obtain peace, O king, an act of great religious merit will be mine, productive of great fruits.
I shall then also save from the meshes of death the Kurus and the Srinjayas inflamed with wrath, the Pandavas and the Dhritarashtras, and, in fact, this entire earth.'
Once there he attempts to convince Duryodhana to give back the kingdom to Pandavas.
Look at thy sons, thy brothers, kinsmen, and other relatives. Let not these chiefs of Bharata's race all perish on thy account. Let not the race of Kauravas be exterminated or reduced. O king, let not people say that thou art the exterminator of thy race and the destroyer of its achievements.
Those mighty car-warriors, the Pandavas (if peace be made) will install thee as the Yuvaraja, and thy father Dhritarashtra, that lord of men, as the sovereign of this extensive empire. Do not, O sire, disregard the prosperity that is awaiting thee and is sure to come.
Giving to the sons of Pritha half the kingdom, win thou great prosperity. Making peace with the Pandavas and acting according to the counsels of thy friends, and rejoicing with them, thou art sure to obtain what is for thy good for ever and ever.'"
But despite all efforts, Duryodhana does not agree.
That share in the kingdom which was formerly given them by my father shall never again, O Kesava, be obtainable by them as long as I live.
As long, O Janardana, as king Dhritarashtra liveth, both ourselves and they, sheathing our weapons, O Madhava, should live in dependence on him.
Given away formerly from ignorance or fear, when I was a child and dependent on others, the kingdom, O Janardana, incapable of being given away again, shall not, O delighter of Vrishni's race, be obtainable by the Pandavas.
At present, O Kesava of mighty arms, as long as I live, even that much of our land which may be covered by the point of a sharp needle shall not, O Madhava, be given by us unto the Pandavas.'"
Frustrated Krishna suggests Dhritrashtra tie Duryodhana and his cronies up and hand them over to the Pandavas.
Hearing these words of Bhishma, the lotus-eyed hero of Dasarha's race, possessed of great powers, addressing all those (that were still there) headed by Bhishma and Drona, said, 'Even this is great transgression, of which all the elders of the Kuru race are becoming guilty, for they do not forcibly seize and bind this wicked king in the enjoyment of sovereignty.
Binding in the same way Duryodhana and Karna and Sakuni, the son of Suvala, and Dussasana, make them over to the Pandavas. For the sake of a family, an individual may be sacrificed. [...] O monarch, binding Duryodhana fast, make peace with the Pandavas. O bull among Kshatriyas, let not the whole Kshatriya race be slaughtered on thy account.'"
Thoroughly annoyed Duryodhana attempts to imprison Krishna to demoralize the Pandavas.
'This Janardana, quick in action, seeketh, with the king Dhritarashtra and Santanu's son, to seize us first. We, however, shall forcibly seize this tiger among men, Hrishikesa, first, like Indra forcibly seizing Virochana's son (Bali). Hearing that this one of Vrishni's race hath been seized, the Pandavas will lose their heart and become incapable of exertion, like snakes whose fangs have been broken. This mighty-armed one is, indeed, the refuge and protection of them all.
However, his plan fails as Krishna unveils his Virat Roop.
Kesava, that slayer of hostile divisions, endued with great energy, addressed Dhritarashtra's son, Duryodhana, and said, 'From delusion, O Suyodhana, thou regardest me to be alone, and it is for this, O thou of little understanding, that thou seekest to make me a captive after vanquishing me by violence.
Here, however, are all the Pandavas and all the Vrishnis and Andhakas. Here are all the Adityas, the Rudras, and the Vasus, with all the great Rishis.
Saying this Kesava, that slayer of hostile heroes burst out into a loud laughter.
And as the high-souled Sauri laughed, from his body, that resembled a blazing fire, issued myriads of gods, each of lightning effulgence, and not bigger than the thumb.
And on his forehead appeared Brahman, and on his breast Rudra. And on his arms appeared the regents of the world, and from his mouth issued Agni, the Adityas, the Sadhyas, the Vasus, the Aswins, the Marutas, with Indra, and the Viswedevas.
And myriads of Yakshas, and the Gandharvas, and Rakshasas also, of the same measure and form, issued thence. And from his two arms issued Sankarshana and Dhananjaya. And Arjuna stood on his right, bow in hand, and (Bala)Rama stood on his left, armed with the plough.
And behind him stood Bhima, and Yudhishthira, and the two sons of Madri, and before him were all the Andhakas and the Vrishnis with Pradyumna and other chiefs bearing mighty weapons upraised.
And on his diverse arms were seen the conch, the discus, the mace, the bow called Saranga, the plough, the javelin, the Nandaka, and every other weapon, all shining with effulgence, and upraised for striking.
And from his eyes and nose and ears and every part of his body, issued fierce sparks of fire mixed with smoke. And from the pores of his body issued sparks of fire like unto the rays of the sun.
And beholding that awful form of the high-souled Kesava, all the kings closed their eyes with affrighted hearts, except Drona, and Bhishma, and Vidura, endued with great intelligence, greatly blessed Sanjaya, and the Rishis, possessed of wealth of asceticism, for the divine Janardana gave unto them this divine sight on the occasion.
And beholding in the (Kuru) court that highly wonderful sight, celestial drums beat (in the sky) and a floral shower fell (upon him). And the whole Earth trembled (at the time) and the oceans were agitated. And, O bull of the Bharata's race, all the denizens of the earth were filled with great wonder.
After it, Krishna leaves the assembly with Satyaki and others.
Then that tiger among men, that chastiser of foes, withdrew that divine and highly wonderful, and extremely varied and auspicious form. And arm-in-arm with Satyaki on one side and Hridika's son (Kritavarman) on the other, and obtaining the permission of the Rishis, the slayer of Madhu went out.