Vishnupurana 3.5.30

Thus eulogized by Yájnawalkya, the sun, in the form of a horse, appeared to him, and said, "Demand what you desire." To which the sage, having prostrated himself before the lord of day, replied, "Give me a knowledge of those texts of the Yajush with which even my preceptor is unacquainted." Accordingly the sun imparted to him the texts of the Yajush called Ayátayáma (unstudied), which were unknown to Vaiśampáyana: and because these were revealed by the sun in the form of a horse, the Brahmans who study this portion of the Yajush are called Vájis (horses). Fifteen branches of this school sprang from Kańwa and other pupils of Yájnawalkya.

Thus eulogized by Yájnawalkya, the sun, in the form of a horse, appeared to him, and said, "Demand what you desire." To which the sage, having prostrated himself before the lord of day, replied, "Give me a knowledge of those texts of the Yajush with which even my preceptor is unacquainted." Accordingly the sun imparted to him the texts of the Yajush called Ayátayáma (unstudied), which were unknown to Vaiśampáyana: and because these were revealed by the sun in the form of a horse, the Brahmans who study this portion of the Yajush are called Vájis (horses). Fifteen branches of this school sprang from Kańwa and other pupils of Yájnawalkya.