Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.1.3

4.3.1.3

sa vai sanne 'cāvāke | ṛtugrahaiścarati tadyatsanne 'cāvāka ṛtugrahaiścarati mithunaṃ vā acāvāka aindrāgno hyacāvāko dvau hīndrāgnī dvandvaṃ hi mithunam prajananaṃ sa etasmānmithunātprajananādṛtūntsaṃvatsaraṃ prajanayati

When the Akhâvâka has (again) taken his seat (behind his hearth), he (his Adhvaryu) proceeds with the libations of the seasons (Ritugraha). The reason why he proceeds with the libations of the seasons when the Akhâvâka is seated, is that the Akhâvâka represents a sexual union, since the Akhâvâka belongs to Indra and Agni, and Indra and Agni are two, and a productive union means a pair: from that same productive union he produces the seasons, the year.