Viṣṇu Purāṇa (2.3):
The Sun’s wife needs a break now and then. Her husband is hot – literally. So she regularly goes off to recuperate and build up her strength again to deal with his immense heat. She didn’t want to depress or insult her husband so, being a goddess and no ordinary woman, she came up with the idea to make a clone of herself to take care of normal affairs in her absence. She hoped the Sun god would not even notice her absence.
Sūrya (the Sun god) already had three children with her: Manu (from whom came human beings), Yama (lord of death) and Yamī (the river Yamuna). Now, while his wife was away, he made three more children with her clone: Śani (the god of Saturn), a second Manu, and Tapatī (another river).
All was well and good until one day the clone-wife got really angry and cursed her “son” Yama. Sūrya realized, “This can’t be Yama’s mother. A mother would never curse her own son so harshly.” Sūrya took the form of a horse and tracked his real wife to a forest where she was meditating for recuperation. She also took the form of a horse and tried to flee. Surya caught up to her and, as horses, they had a third batch of three children: The Aśvinī twins and a son named Revanta.