Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 73
Sanskrit:
सूत उवाचएवं स्तुत: स भगवान् वाजिरूपधरो रवि: ।यजूंष्ययातयामानि मुनयेऽदात् प्रसादित: ॥ ७३ ॥
ITRANS:
sūta uvācaevaṁ stutaḥ sa bhagavānvāji-rūpa-dharo raviḥyajūṁṣy ayāta-yāmānimunaye ’dāt prasāditaḥ
Translation:
Sūta Gosvāmī said: Satisfied by such glorification, the powerful sun-god assumed the form of a horse and presented to the sage Yājñavalkya yajur-mantras previously unknown in human society.