Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.2

3.4.1.2

atha yasmādātithyaṃ nāma | atithirvā eṣa etasyāgacati yatsomaḥ krītastasmā etadyathā rājñe vā brāhmaṇāya vā mahokṣaṃ vā mahājaṃ vā pacettadaha mānuṣaṃ havirdevānāmevamasmā etadātithyaṃ karoti

Now as to why it is called 'guest-offering.' He, the purchased Soma, truly comes as his (the sacrificer's) guest,--to him (is offered) that (hospitable reception): even as for a king or a Brâhman one would cook a large ox or a large he-goat--for that is human (fare offered to a guest), and the oblation is that of the gods--so he prepares for him that guest-offering.