Śatapatha Brāhmaṇa ("of a hundred paths") 14.2.2.53

14.2.2.53

tadāhuḥ yadvānaspatyairdevebhyo juhvatyatha kasmādetam mṃmayenaiva juhotīti yajñasya śīrṣacinnasya raso vyakṣaratsa ime dyāvāpṛthivī agacadyanmṛdiyaṃ tadyadāpo'sau tanmṛdaścāpāṃ ca mahāvīrāḥ kṛtā bhavanti tenaivainametadrasena samardhayati kṛtsnaṃ karoti

And, again, they say, 'Seeing that they make offering to the gods by means of wooden (vessels), wherefore does he offer this (Gharma) by means of one made of clay?' When the Sacrifice had its head cut off, its life-sap flowed away and entered the heaven and the earth. Now this (earth) is clay, and yonder (sky) is water; and the Mahâvîra (vessels) are made of clay and water: thus he supplies and completes it (the Pravargya) with that life-sap.