Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.3.22

3.4.3.22

athāha samullupya prastaram | agnīnmadantyāpāiti madantītyagnīdāha tābhirehītyuparyuparyagnimatiharati sa yannānupraharatyetena hyata ūrdhvānyahāni pracariṣyanbhavatyatha yaduparyuparyagnimatiharati tadevāsyānuprahṛtabhājanam bhavati tamagnīdhe prayacati tamagnīnnidadhāti

Having then picked up the prastara, he says, 'Agnîdh, does the water boil? It boils,' replies the Agnîdh. 'Come hither with it!' He holds (the prastara) quite close over the fire. The reason why he does not throw it into the fire is that he (the sacrificer) is to perform therewith during the days that follow; and in that he holds it quite close over the fire, thereby it is for him as if it were really thrown into the fire. He hands it to the Agnîdh, and the Agnîdh puts it aside (in a safe place).