Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.2.7

9.2.2.7

yadvevaitāḥ samidha ādadhāti | etadvā enaṃ devāḥ sarvaṃ kṛtsnaṃ saṃskṛtyāthainametenānnenāprīṇannetābhiḥ samidbhistathaivainamayametatsarvaṃ kṛtsnaṃ saṃskṛtyāthainametenānnena prīṇātyetābhiḥ samidbhiraudumbaryo bhavantyārdrā ghṛte nyuttā sarvāṃ rātriṃ vasanti tasyokto bandhurudenamuttarāṃ nayendremam pratarāṃ naya yasya kurmo gṛhe haviriti yathā yajustathā bandhustisraḥ samidha ādadhāti trivṛdagniryāvānagniryāvatyasya mātrā tāvataivainametadannena prīṇāti tisa āhutīrjuhoti tatṣaṭ tasyokto bandhuḥ

And, again, as to why he puts the pieces of firewood on; the gods having set him up wholly and completely, now regaled him with this food, these pieces of firewood; and in like manner does this (Sacrificer), now that he has set him up wholly and completely, regale him with this food, these pieces of firewood. They are of udumbara wood, and fresh, and remain for a whole night (being) soaked in ghee: the significance of this has been explained. [He puts them on, with, Vâg. S. XVII, 50-52], 'Upwards lead thou him, O Agni!... Forward lead thou him, O Indra!... In whose house we make offering...,' as the text, so the meaning. Three pieces of firewood he puts on,--threefold is Agni: as great as Agni is, as great as is his measure, with so much food he thus regales him. Three oblations he offers,--that makes six: the significance of this has been explained.