Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.4.12

1.4.4.12

āsīnastamāghārayati | yo mūlaṃ yajñasya niṣaṇamiva hīdam mūlaṃ tiṣṭhaṃstamāghārayati yaḥ śiro yajñasya tiṣṭhatīva hīdaṃ śiraḥ

Sitting he makes that libation which is the root of the sacrifice; for seated, as it were, is this root. Standing he makes that libation which is the head of the sacrifice; for this head stands, as it were.