Mbh 5.180.5

“‘Bhishma continued, “While, O king, Rama and Amva were talking thus with each other, the Rishi (Akritavrana) of highly virtuous soul said these words, ‘It behoveth thee not, O mighty-armed one, to desert this girl that seeketh thy protection! If summoned to battle, Bhishma cometh to the encounter and sayeth--“I am vanquished,” or, if he obeyeth thy words, then that which this maiden seeketh will be accomplished, O son of Bhrigu’s race, and the words spoken by thee, O hero, will also, O lord, be true! This also was, O great Muni, the vow then made by thee, O Rama,--the vow made by thee before Brahmanas after thou hadst conquered all the Kshatriyas, viz., that thou wouldst slay in battle the person, be he a Brahmana, a Kshatriya, a Vaisya, or a Sudra, who would be a foe to the Brahmanas. Thou hadst further promised that as long as thou wouldst live thou wouldst not abandon those that would come to thee in fright and seek thy protection, and that thou wouldst, O Bhargava, slay that proud warrior who would vanquish in battle all the assembled Kshatriyas of the earth! O Rama, even Bhishma, that perpetuator of Kuru’s race, hath achieved such success (over all the Kshatriyas)! Approaching him, O son of Bhrigu’s race, encounter him now in battle!’