Śatapatha Brāhmaṇa ("of a hundred paths") 2.2.2.15

2.2.2.15

tadyatrainamado manthanti | tajjātamabhiprāṇiti prāṇo vā agnirjātamevainametatsantaṃ janayati sa punarapāniti tadenamantarātmannādhatte so 'syaiṣo 'ntarātmannagnirāhito bhavati

Now, when, on that occasion, they produce that (fire) by churning, then he (the sacrificer) breathes (blows) upon it, when produced; for fire indeed is breath: he thereby produces the one thus produced. He again draws in his breath: thereby he establishes that (fire) in his innermost soul; and that fire thus becomes established in his innermost soul.