Mbh 12.55.3

"'Bhishma said, "As the duty of the Brahmanas consists of the practice of charity, study, and penances, so the duty of Kshatriyas is to cast away their bodies, O Krishna, in battle. A Kshatriya should slay sires and grandsires and brothers and preceptors and relatives and kinsmen that may engage with him in unjust battle. This is their declared duty. That Kshatriya, O Kesava, is said to be acquainted with his duty who slays in battle his very preceptors if they happen to be sinful and covetous and disregardful of restraints and vows. That Kshatriya is said to be acquainted with his duty who slays in battle the person that from covetousness disregards the eternal barriers of virtue.[160] That Kshatriya is said to be acquainted with duty who in battle makes the earth a lake of blood, having the hair of slain warriors for the grass and straw floating on it, and having elephants for its rocks, and standards for the trees on its banks. A Kshatriya, when challenged, should always fight in battle, since Manu has said that a righteous battle (in the case of a Kshatriya) leads to both heaven and fame on earth."'