Mbh 6.108.7

“Vasudeva said, ‘Having vowed the slaughter of Bhishma before, O Jishnu, how canst thou abstain from slaying him, agreeably to the duties of a Kshatriya? Throw down from his car, O Partha, that Kshatriya who is invincible in battle. Victory can never be yours without slaying Ganga’s son. Even thus shall he go to the abode of Yama. This hath been settled before by the gods. That which hath been destined before, O Partha, must happen. It cannot be otherwise. None save thee, O invincible one, not even the wielder of the thunder-bolt himself, would be capable of fighting with Bhishma, who is like the Destroyer with wide-open mouth. Slay Bhishma, without any anxiety. Listen also to these words of mine that are what Vrihaspati of great intelligence had said unto Sakra in days of old. One should slay even an aged person endued with every merit and worthy of reverence if he cometh as a foe, or, indeed any other who approacheth for destroying one’s self--O Dhananjaya, this is the eternal duty sanctioned for the Kshatriya, viz., that they should fight, protect subjects, and perform sacrifices, all without malice.’