Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.22

7.3.1.22

atha paścāt | agne yatte śukraṃ yaccandraṃ yatpūtaṃ yacca yajñiyamitīyaṃ vā agnirasyai tadāha taddevebhyo bharāmasīti tadasmai daivāya karmaṇe harāma ityetattāmāhṛtyāntareṇa pucasaṃdhimātmannupadadhāti sa yaḥ pratīcyāṃ diśi raso 'tyakṣarattamasminnetaddadhātyatho pratīcīmevāsminnetaddiśaṃ dadhāti sa na samprati paścādāharennedyajñapathādrasamāharāṇītīta ivāharati

Then from behind (he fetches one, with Vâg. S. XII, 104), 'O Agni, what in thee is pure, what brilliant, what clean, what meet for sacrifice,'--Agni doubtless is this (earth): of her he says this;--'that do we bring to the gods,' that is, 'that we bring for this divine work.' Having brought it he puts it on the body (of the altar) inside the junction of the tail (and the body): he thereby puts into him what vital sap had flowed away from him in the western direction, and also the western region itself he bestows upon him. Let him not take it exactly from the back (west) lest he should take the vital sap from the path of the sacrifice: he takes it from about there.