Shatapatha Brahmana 13.6.1.9

13.6.1.9

yavamadhyaḥ pañcarātro bhavati ime vai lokāḥ puruṣamedha ubhayatojyotiṣo vā ime lokā agnineta ādityenāmutastasmādubhayatojyotirannamukthya ātmātirātrastadyadetā ukthyāvatirātramabhito bhavatastasmādayamātmānnena parivṛḍho'tha yadeṣa varṣiṣṭo'tirātro'hnāṃ sa madhye tasmādyavamadhyo yute ha vai dviṣantam bhrātrṛvyamayamevāsti nāsya dviṣanbhrātṛvya ityāhurya evam veda

It is a five-days’ (sacrifice), like a barley-corn in the middle; for the Purushamedha is these worlds, and these worlds have light on both sides--through Agni (the sacrificial fire) on this side, and through Âditya (the Sun) on the other: therefore it has light on both sides. And the Ukthya is food, and the Atirâtra the body (self); and because there are these Ukthyas on both sides of the Atirâtra, therefore this body is surrounded by food. And that Atirâtra which is the largest of them is in. the middle, it is thereby that it (the body, or sacrifice) is like a barley-corn (yava) in the middle; and, verily, whosoever knows this repels (yu) his hateful enemy: 'He alone exists, not his hateful enemy,' thus they say of him.