Shatapatha Brahmana 4.3.1.18

4.3.1.18

upayāmagṛhīto 'si sahase tvetyevādhvaryurgṛhṇātyupayāmagṛhīto 'si sahasyāya tveti pratiprasthātaitāveva haimantikau sa yaddhemanta imāḥ prajāḥ sahaseva svaṃ vaśamupanayate teno haitau sahaśca sahasyaśca

With 'Thou art taken with a support: thee for Sahas!' the Adhvaryu draws (the ninth); with 'Thou art taken with a support: thee for Sahasya!' the Pratiprasthâtri (the tenth). These two are the winter (months): because the winter by force (sahas) brings these creatures into his power, therefore these two are Saha and Sahasya.