Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.1.17

3.4.1.17

āśvavālaḥ prastaraḥ | yajño ha devebhyo 'pacakrāma so 'śvo bhūtvā parāṅāvavarta tasya devā anuhāya vālānabhipedustānālulupustānālupya sārdhaṃ saṃnyāsustata etā oṣadhayaḥ samabhavanyadaśvavālāḥ śiro vai yajñasyātithyaṃ jaghanārdho vālā ubhayata evaitadyajñam parigṛhṇāti yadāśvavālāḥ prastaro bhavati

The prastara-bunch is of asvavâla-grass (Saccharum Spontaneum). For, once upon a time, the sacrifice escaped from the gods. It became a horse (asva) and sped away from them. The gods, rushing after it, took hold of its tail (vâla) and tore it out; and having torn it out, they threw it down in a lump, and what had been the hairs of the horse's tail then grew up as those plants (of asvavâla-grass). Now the guest-offering is the head of the sacrifice, and the tail is the hind-part (of animals): hence by the prastara being of asvavâla-grass he encompasses the sacrifice on both sides.