Shatapatha Brahmana 4.5.9.4

4.5.9.4

atha ṣaṣṭhe 'hanvyūhati | grahānvyūhanti candāṃsi tacukrāgrān gṛhṇātyaindraṃ vā etatṣaṣṭhamaharbhavatyeṣa vai śukro ya eṣa tapatyeṣa u evendrastasmācukrāgrān gṛhṇāti

Thereupon, on the sixth day, he transposes the grahas, and they transpose the metres. There he draws the cups beginning with the Sukra; for that sixth day is Indra's own, and the Sukra (bright, clear) is he that burns yonder, and he (the sun) indeed is Indra: therefore he draws the cups beginning with the Sukra.