Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.31

3.8.3.31

tadyadantareṇa | ardharcau yājyāyai vasāhomaṃ juhotīyaṃ vā ardharco 'sau dyaurardharco 'ntarā vai dyāvāpṛthivī antarikṣamantarikṣāya vai juhoti tasmādantareṇārdharcau yājyāyai vasāhomaṃ juhoti

And as to why he offers the oblation of gravy in the interval between the two half-verses of the offering-prayer,--one half-verse, forsooth, is this earth, and the other half-verse is yonder sky. Now between the sky and the earth is the air, and it is to the air that he offers: therefore he offers the oblation of gravy between the two half-verses of the offering-prayer.