Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.2.24

7.5.2.24

yadvevainadabhijuhoti | śīrṣaṃstadvīryaṃ dadhātyājyena juhoti vajro vā ājyaṃ vīryaṃ vai vajro vīryamevāsminnetaddadhāti svāhākāreṇa vṛṣā vai svāhākāro vīryaṃ vai vṛṣā vīryamevāsminnetaddadhāti priṣṭubhā vajro vai triṣṭubvīryaṃ vai vajro vīryaṃ triṣṭubvīryeṇaivāsminnetadvīryaṃ dadhāti

And, again, why he offers thereon:--he thereby lays vigour into the head. He offers with ghee,--ghee is a thunderbolt, and the thunderbolt means vigour: he thus lays vigour into it. With 'Hail' (he offers),--the 'Hail' (svâhâkâra, m.) is a male, and the male means vigour: he thus lays vigour into it. With a trishtubh verse (he offers);the Trishtubh is a thunderbolt, and the thunderbolt means vigour; the Trishtubh is vigour: with vigour he thus lays vigour into it.