Shatapatha Brahmana 1.4.5.5

1.4.5.5

athāsaṃsparśayantsrucau paryetya | dhruvayā samanakti śiro vai yajñasyottara āghāra ātmā vai dhruvā tadātmanyevaitaciraḥ pratidadhāti śiro vai yajñasyottara āghāraḥ śrīrvai śiraḥ śrīrhi vai śirastasmādyo 'rdhasya śreṣṭho bhavatyasāvamuṣyārdhasya śira ityāhuḥ

Having then returned (to his former position behind the altar), without letting the two offering-spoons touch each other, he mixes (some of the butter left in the guhû) with (that in) the dhruvâ. Now the second libation (which he has just offered) is the head of the sacrifice, and the dhruvâ is its body: hence he thereby replaces the head on the body. And the second libation, moreover, is the head of the sacrifice, and the head (siras) represents excellence (srî), for the head does indeed represent excellence: hence, of one who is the most excellent (sreshtha) of a community, people say that he is 'the head of that community.'