Shatapatha Brahmana 9.1.2.14

9.1.2.14

āsīnaḥ kurvītetyu haika āhuḥ | āsīno vai dhenuṃ dogdhīti tiṣṭhaṃstveva kurvīteme vai lokā eṣo 'gnistiṣṭhantīva vā ime lokā atho tiṣṭhanvai vīryavattaraḥ

And some say, 'Let him make them his own whilst sitting, for sitting one milks the cow.' But let him rather make them his own standing; for that fire-altar is these worlds, and these worlds are, as it were, standing. And, moreover, one is stronger whilst standing.