Mahabharata 3.165.2

3.165.2

na tvamadya yudhā jetuṃ śakyaḥ suragaṇairapi |
kiṃ punarmānuṣe loke mānuṣairakṛtātmabhiḥ |
aprameyo'pradhṛṣyaśca yuddheṣvapratimastathā || 2 ||

"And when I had acquired proficiency in weapons, and gained his confidence that one having for his vehicle the horse (Uchchaisrava), (Indra), patting me on the head with his hand, said these words, 'Now even the celestials themselves cannot conquer thee,--what shall I say of imperfect mortals residing on earth? Thou hast become invulnerable in strength, irrepressible, and incomparable in fight.' Then with the hair of his body standing on end, he again accosted me saying, 'O hero, in fighting with weapons none is equal unto thee. And, O perpetuator of the Kuru race, thou art even watchful, and dexterous, and truthful, and of subdued senses, and the protector of the Brahmanas and adept in weapons, and warlike. And, O Partha, together with (a knowledge of) the five modes, using (them), thou hast obtained five and ten weapons and, therefore, there existeth none, who is thy peer. And thou hast perfectly learnt the discharge (of those weapons) and (their) withdrawal, and (their) re-discharge and re-withdrawal, and the Prayaschitta connected (with them), and also their revival, in case of their being baffled. Now, O represser of foes, the time hath arrived for thy paying the preceptor's fee. Do thou promise to pay the fee; then I shall unfold unto thee what thou wilt have to perform.' Thereat, O king, I said unto the ruler of the celestials, 'If it be in my power to do the work, do thou consider it as already accomplished by me.' O king, when I had said these words, Indra with a smile said unto me 'Nothing is there in the three worlds that is not in thy power (to achieve) My enemies, those Danavas, named, Nivata-Kavachas dwell in the womb of the ocean. And they number thirty million and are notorious, and all of equal forms and strength and splendour. Do thou slay them there, O Kunti's son; and that will be thy preceptor's fee.'