Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.29

2.3.1.29

sā yā pūrvāhutiḥ | sātmānamabhi hūyate tām mantreṇa juhotyaddhā hi tadyanmantro 'ddho tadyadātmā 'tha yottarā sā prajāmabhi hūyate tāṃ tūṣṇīṃ juhotyanaddhā hi tadyattūṣṇīmanaddho tadyatprajā

Now that first libation is offered on account of the self: he offers it with a sacred text, for certain is the sacred text, and certain also is the self. And that second one is offered on account of progeny: he offers it silently, for uncertain is what (is done) silently, and uncertain also is progeny.