Shatapatha Brahmana 4.3.1.19

4.3.1.19

upayāmagṛhīto 'si tapase tvetyevādhvaryurgṛhṇātyupayāmagṛhīto 'si tapasyāya tveti pratiprasthātaitāveva śaiśirau sa yadetayorbaliṣṭhaṃ śyāyati teno haitau tapaśca tapasyaśca

With 'Thou art taken with a support: thee for Tapas!' the Adhvaryu draws (the eleventh); with 'Thou art taken with a support: thee for Tapasya!' the Pratiprasthâtri (the twelfth). These two are (the months) of the dewy season: because during them it freezes most severely, therefore these two are Tapas and Tapasya.