Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.2.1

1.3.2.1

puruṣo vai yajñaḥ | puruṣastena yajño yadenam puruṣastanuta eṣa vai tāyamāno yāvāneva puruṣastāvānvidhīyate tasmātpuruṣo yajñaḥ

Now the sacrifice is the man. The sacrifice is the man for the reason that the man spreads (performs) it; and that in being spread it is made of exactly the same extent as the man: this is the reason why the sacrifice is the man.