Mbh 13.12.2a

"'Bhishma said, "In this connection is cited this old narrative of the discourse between Bhangaswana and Sakra as a precedent illustrating the question. In days of yore there lived a king of the name of Bhangaswana. He was exceedingly righteous and was known as a royal sage. He was, however, childless, O chief of men, and therefore performed a sacrifice from desire of obtaining an issue. The sacrifice which that mighty monarch performed was the Agnishtuta. In consequence of the fact that the deity of fire is alone adored in that sacrifice, this is always disliked by Indra. Yet it is the sacrifice that is desired by men when for the purpose of obtaining an issue they seek to cleanse themselves of their sins.[30] The highly blessed chief of the celestials, viz. Indra, learning that the monarch was desirous of performing the Agnishtuta, began from that moment to look for the laches of that royal sage of well-restrained soul (for if he could succeed in finding some laches, he could then punish his disregarder). Notwithstanding all his vigilance, however, O king, Indra failed to detect any laches, on the part of the high-souled monarch. Some time after, one day, the king went on a hunting expedition. Saying unto himself--This, indeed, is an opportunity,--Indra stupefied the monarch. The king proceeded alone on his horse, confounded because of the chief of the celestials having stupefied his senses. Afflicted with hunger and thirst, the king's confusion was so great that he could not ascertain the points of the compass. Indeed, afflicted with thirst, he began to wander hither and thither. He then beheld a lake that was exceedingly beautiful and was full of transparent water. Alighting from his steed, and plunging into the lake, he caused his animal to drink. Tying his horse then, whose thirst had been slaked, to a tree, the king plunged into the lake again for performing his ablutions. To his amazement he found that he was changed, by virtue of the waters, into a woman.