Śatapatha Brāhmaṇa ("of a hundred paths") 8.7.1.5

8.7.1.5

sa upadadhāti | tapaśca tapasyaśca śaiśirāvṛtū iti nāmanī enayorete nāmabhyāmevaine etadupadadhātyasau vā ādityastapastasmādetāvṛtū anantarhitau tadyadetasmādetāvṛtū anantarhitau tasmādetau tapaśca tapasyaśca

He lays them down, with (Vâg. S. XV, 57), 'Tapa and Tapasya, the two dewy seasons,'--these are the names of these two: it is thus by their names that he lays them down. Tapa (the burner), doubtless, is yonder sun: from him these two seasons are not separated; and inasmuch as these two seasons are not separated from him, they are called Tapa and Tapasya.