Shatapatha Brahmana 9.1.1.40

9.1.1.40

triṣkṛtvaḥ pratyavarohati | trivṛdagniryāvānagniryāvatyasya mātrā tāvataivainānetadannena prīṇāti svāhākāreṇānnaṃ vai svāhākāro 'nnenaivainānetatprīṇāti tririti ūrdhvo rohati tatṣaṭ tasyokto bandhuḥ

In three turns he descends,--Agni is threefold: as great as Agni is, as great as is his measure, by so much food he thus gratifies them. With 'Svâhâ' (he makes offering), for the Svâhâ is food: with food he thus gratifies them. Thrice (in three turns) he ascends from hence upwards,--that makes six: the significance of this has been explained.