Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.4.20

4.2.4.20

taṃ vai gopāyanti | śiro vā eṣa etasyai gāyatryai yajño vai gāyatrī dvādaśa stotrāṇi dvādaśa śastrāṇi taccaturviṃśatiścaturviṃśatyakṣarā vai gāyatrī tasyā eṣa śiraḥ śrīrvai śiraḥ śrīrhi vai śirastasmādyo 'rdhasya śreṣṭo bhavatyasāvamuṣyārdhasya śira ityāhuḥ śreṣṭho ha vyatheta yadeṣa vyatheta yajamāno vai śreṣṭho nedyajamāno vyathātā iti tasmādvai gopāyanti

They indeed watch over it; for this (cup of Soma) is the head of Gâyatrî, Gâyatrî being the sacrifice,--there are twelve chants (stotra) and twelve recitations (sastra): that makes twenty-four, and of twenty-four syllables consists the Gâyatrî. This cup of Soma is her head; but the head means excellence, for the head indeed means excellence: hence people say of him who is the best man of a place, that 'so and so is the head of such and such a place.' And, indeed, the best man would come to harm, if this (cup) were to come to harm; and, the best man being the sacrificer, they watch (this cup) lest the sacrificer should come to harm.