Shatapatha Brahmana 4.3.1.16

4.3.1.16

upayāmagṛhīto 'si nabhase tvetyevādhvaryurgṛhṇātyupayāmagṛhīto 'si nabhasyāya tveti pratiprasthātaitāveva vārṣikāvamuto vai divo varṣati teno haitau nabhaśca nabhasyaśca

With 'Thou art taken with a support: thee for Nabhas!' the Adhvaryu draws (the fifth); with 'Thou art taken with a support: thee for Nabhasya!' the Pratiprasthâtri (the sixth). These two are (the months) of the rainy season: it rains from yonder sky, and hence these two are Nabhas (mist, cloud) and Nabhasya.