Bhagavatapurana 12.6.74

12.6.74

yajurbhir akaroc chākhā daśa pañca śatair vibhuḥ /
jagṛhur vājasanyas tāḥ kāṇva-mādhyandinādayaḥ //

74. The powerful sage divided them into fifteen branches, each comprising hundreds of Yajus-sentences; and Kanva, Madhyandina and others learnt those branches called Vajasani (so called because they were evolved in great rapidity by Surya in the form of a horse through His manes).