Śatapatha Brāhmaṇa ("of a hundred paths") 11.2.5.2

11.2.5.2

tadāhuḥ pade-pade'śvaśya medhyasyāhutiṃ juhvatīti sa yatsāyamprātaragnihotraṃ juhoti dve sāyamāhutī juhoti dve prātastāścatasra āhutyaścatuṣpādvā aśvastadasya pade-pada evāhutirhutā bhavati

As to this, they say, 'For each foot of the sacrificial horse they offer an oblation;'--when he performs the Agnihotra in the evening and morning, he offers two oblations in the evening, and two in the morning--that makes four oblations: thus--the horse being four-footed--an oblation is offered for each of its feet.