Shatapatha Brahmana 8.1.3.5

8.1.3.5

tadāhuḥ | yadyathā pituḥ putramevaṃ trīṇi prathamānyāhātha kasmādṛkṣāmayoḥ saṃkrāmatīti sāma vā ṛcaḥ patistadyattatrāpi yathā pituḥ putramevam brūyādyathā patiṃ santam putram brūyāttādṛktattasmādṛkṣāmayoḥ saṃkrāmati kasmādu triḥ saṃtanotīti pitaram putram pautraṃ tāṃstatsaṃtanoti tasmādu tebhya eka eva dadāti

As to this they say, 'When he speaks first of three in the same way as of a father's son, how, then, does this correspond as regards the rile and sâman?' The sâman, doubtless, is the husband of the Rik; and hence were he also in their case to speak as of a father's son, it would be as if he spoke of him who is the husband, as of the son: therefore it corresponds as regards the rik and sâman. 'And why does he thrice carry on (the generation from father to son)?'--father, son, and grandson: it is these he thereby carries on; and therefore one and the same (man) offers (food) to them.