Krishna speaks his first line in the Mahabharata in the Adi Parva (Swayamvara Parva) section—the venue is the royal palace where Draupadi’s svayamvara (the ceremony in which a girl of royal bloodline and marriageable age selects her husband from a group of eligible suitors) has been organized. To test the skill, strength, and divine powers of the contestants at the svayamvara, King Draupada, Draupadi’s father, has arranged a device in which a fish revolves above a pool of water. The contestants must string a steel bow, and, while looking into the pond where a reflection of the revolving fish is visible, shoot the fish in the eye with a steel arrow. The one who manages to accomplish this superhuman feat wins Draupadi’s hand in marriage. The five Pandava brothers have arrived at the ceremony disguised as Brahmin priests. When all the contestants fail to string the steel bow and hit the target (or have been disqualified for being lowborn, like Karna), one of the Pandava bothers, Arjuna, picks up the steel bow. He strings it with ease and with gaze fixed on the reflection in the pond, he shoots the steel arrow, which hits the target, the fish’s eye. The kings, princes, demigods, and saints who have arrived at the svayamvara are infuriated to see that a man, who is attired like a Brahmin priest, has managed to accomplish the feat at which all of them have failed. They blame King Draupada for humiliating them by allowing an unknown Brahmin priest to be the winner of the svayamvara. They rush forward to kill him. But to get to King Draupada they have to pass through the Pandava brothers.
Here’s an excerpt from the translation of the verses which describe the struggle between the Pandava brothers and their rivals at Draupadi’s svayamvara:
“Then the monarchs with gloved fingers and upraised weapons rushed in anger at the Kuru princes, Bhima and Arjuna, to slay them. Then the mighty Bhima of extraordinary achievements, endued with the strength of thunder, tore up like an elephant a large tree and divested it of its leaves. And with that tree, the strong-armed Bhima, the son of Pritha, that grinder of foes, stood, like unto the mace-bearing king of the dead (Yama) armed with his fierce mace, near Arjuna that bull amongst men. And beholding that feat of his brother, Jishnu of extraordinary intelligence, himself also of inconceivable feats, wondered much. And equal unto Indra himself in achievements, shaking off all fear he stood with his bow ready to receive those assailants.”
At this point, Krishan speaks his first line in the Mahabharata. He is addressing his brother, Balarama, who is pointing out Bhima and Arjuna:
“And beholding those feats of both Jishnu and his brother, Damodara (Krishna) of superhuman intelligence and inconceivable feats, addressing his brother, Halayudha (Valadeva) of fierce energy, said, 'That hero there, of tread like that of a mighty lion, who draweth the large bow in his hand four full cubits in length, is Arjuna! There is no doubt, O Sankarshana, about this, if I am Vasudeva. That other hero who having speedily torn up the tree hath suddenly become ready to drive off the monarchs is Vrikodara! For no one in the world, except Vrikodara, could today perform such a feat in the field of battle. And that other youth of eyes like unto lotus-petals, of full four cubits height, of gait like that of a mighty lion, and humble withal, of fair complexion and prominent and shining nose, who had, a little before, left the amphitheatre, is Dharma's son (Yudhishthira). The two other youths, like unto Kartikeya, are, I suspect, the sons of the twin Aswins. I heard that the sons of Pandu along with their mother Pritha had all escaped from the conflagration of the house of lac.' Then Halayudha of complexion like unto that of clouds uncharged with rain, addressing his younger brother (Krishna), said with great satisfaction, 'O, I am happy to hear, as I do from sheer good fortune, that our father's sister Pritha with the foremost of the Kaurava princes have all escaped (from death)!’"