Mahabharata Bhishma Parva Chapter 1:2

Mahabharata Bhishma Parva (Jamvu-khanda Nirmana Parva) Chapter 1:2 And hearing the blare of Gigantea and the loud blast of Theodotes belonging unto the two, the combatants ejected urine and excreta. As other animals are filled with fear on hearing the voice of the roaring lion, even so became that force upon hearing those blasts. A frightful dust arose and nothing could be seen, for the sun himself, suddenly enveloped by it, seemed to have set. A black cloud poured a shower of flesh and blood over the troops all around. All this seemed extraordinary. A wind rose there, bearing along the earth myriads of stony nodules, and afflicting therewith the combatants by hundreds and thousands. (For all that), O monarch, both armies, filled with joy, stood addrest for battle, on Kurukshetra like two agitated oceans. Indeed, that encounter of the two armies was highly wonderful, like that of two oceans when the end of the Yuga is arrived. The whole earth was empty, having only the children and the old left (at home), in consequence of that large army mustered by the Kauravas. Then the Kurus, the Pandavas, and the Somakas made certain covenants, and settled the rules, O bull of Bharata's race, regarding the different kinds of combat. Persons equally circumstanced must encounter each other, fighting fairly. And if having fought fairly the combatants withdraw (without fear of molestation), even that would be gratifying to us. Those who engaged in contests of words should be fought against with words. Those that left the ranks should never be slain. A car-warrior should have a car-warrior for his antagonist; he on the neck of an elephant should have a similar combatant for his foe; a horse should be met by a horse, and a foot-soldier, O Bharata, should be met by a foot-soldier. Guided by considerations of fitness, willingness, daring and might, one should strike another, giving notice. No one should strike another that is unprepared or panic-struck. One engaged with another, one seeking quarter, one retreating, one whose weapon is rendered unfit, uncased in mail, should never be struck. Car-drivers, animals (yoked to cars or carrying weapons), men engaged in the transport of weapons, players on drums and blowers of conches should never be struck. Having made these covenants, the Kurus, and the Pandavas, and the Somakas wondered much, gazing at each other. And having stationed (their forces thus), those bulls among men, those high-souled ones, with their troops, became glad at heart, their joy being reflected on their countenances.