Mahabharata 4.2.25

अजुन उवाच प्रतिज्ञां पण्ढको5स्मीति करिष्यामि महीपते । ज्याघातौ हि महान्तौ मे संवर्तु नृप दुष्करी

arjuna uvāca pratijñāṃ paṇḍako ’smīti kariṣyāmi mahīpate | jyāghātau hi mahāntau me saṃvartuṃ nṛpa duṣkarī ||

Arjuna said: “O lord of the earth, I shall make the vow that I am a eunuch. Yet, O king, it will be difficult for me to maintain it, for the twang of my bowstring and the force of my bow are great.”