Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.1.15

4.3.1.15

upayāmagṛhīto 'si | śukrāya tvetyevādhvaryurgṛhṇātyupayāmagṛhīto 'si śucaye tveti pratiprasthātaitāveva graiṣmau sa yadetayorbaliṣṭhaṃ tapati teno haitau śukraśca śuciśca

With 'Thou art taken with a support: thee for Sukra!' the Adhvaryu draws (the third); with 'Thou art taken with a support: thee for Suki!' the Pratiprasthâtri (the fourth). These two are the summer (months): because during them it burns fiercest, therefore these two are Sukra (clear) and Suki (bright).