Mahabharata Bhishma Parva Chapter 99:2

Mahabharata Bhishma Parva (Bhagavat-Gita Parva) Chapter 99:2 Slain by them in battle, I will go to Yama's abode, or slaying them in battle, I will give thee joy. Sikhandin was born in Drupada's palace as female at first. She became a male in consequence of the grant of a boon. After all, however, she is Sikhandini. Him I will not slay even if I have to lose my life, O Bharata. She is the same Sikhandini that the Creator had first made her. Pass the night in happy sleep, O son of Gandhari. Tomorrow I will fight a fierce battle about which men will speak as long as the world lasts.' Thus addressed by him, thy son, O monarch, came away. And saluting his signior with a bow of the head, he came back to his own tent. Coming back, the king dismissed his attendants. And soon then that destroyer of foes entered his abode. And having entered (his tent) the monarch passed the night (in sleep). And when the night dawned, rising up, the king, ordered all the royal warriors, saying, 'Draw up the forces. Today Bhishma, excited with wrath, will slay all the Somakas. Hearing those copious lamentations of Duryodhana in the night, Bhishma regarded them, O king, as commands to himself. Filled with great grief and deprecating the status of servitude, Santanu's son reflected for a long time, thinking of an encounter with Arjuna in battle. Understanding from signs that Ganga's son had been thinking of that, Duryodhana, O king, commanding Dussasana, saying, 'O Dussasana, let cars be quickly appointed for protecting Bhishma. Let all the two and twenty divisions (of our army) be urged on. That hath now come about which we had been thinking for a series of years, viz., the slaughter of the Pandavas with all their troops and the acquisition (by ourselves) of the kingdom. In this matter, I think, the protection of Bhishma is our foremost duty. Protected by us, he will protect us and slay the Parthas in battle. Of cleansed soul, he said unto me,—"I will not slay Sikhandini. He was a female before, O king, and, therefore, should be avoided by me in battle. The world knoweth, O thou of mighty arms, that from desire of doing good to my father, I formerly gave up a swelling kingdom. I will not, therefore, slay in battle, O foremost of men, any female or anybody that was a female before. This that I tell thee is true. This Sikhandin, O king, was first born a female. Thou hast heard that story. She was born as Sikhandini after the manner I told thee before the battle began. Taking her birth as a daughter she hath become a man. Indeed, she will fight with me, but I will never shoot my arrows at her. As regards all other Kshatriyas desirous of victory to the Pandavas, O sire, whom I may get within my reach on the field of battle, I will slay them."—These were the words that Ganga's son acquainted with the scriptures, that chief of Bharata's race, said unto me.