Ramayana

After the poet Valmik learned the story of Rama, he went to bathe in a river. By the river a pair of mating cranes were sweetly singing. A Nishada hunter, hostile and ploting evil, shot down the male of the couple. When the hen saw her mate writhing on the ground, his limbs covered in blood. she cried out words of compassion. And when Valmiki saw that the Nishada had brought down the male crane, he was overcome with compassion, and out of his feeling of compassion he thought. “This was not dharma. to kill a sweetly singing crane for no reason.” When he heard the female crane crying, he said, “Nishada, you will never find peace, since you killed the male of this pair of cnnes at the height of his desire.” Then Valmiki realized that he had instinctively spoken in verse. in a meter that he called the shloka, because it was uttered in sorrow (shoka).

-Ramayana (400 BCE to 200CE) {l.2.81.1-17)