In The Visnu Purana it is stated :
The seven horses yoked to the sun-god’s chariot are named Gayatri, Brhati, Usnik, Jagati, Tristup, Anustup and Pankti. These names of various Vedic meters designate the seven horses that carry the sun-god’s chariot.
(Srimad Bhagavatam - Canto 5, CH. 21, Text 15)
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