Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.4.8

4.2.4.8

yadvā asyāvācīnaṃ nābheḥ | tadasyaiṣa ātmanaḥ sa yatpuraitasya homātsarpedvā pra vā srāvayeta dhruvaṃ hāvamehenneddhruvamavamehānīti tasmādvā agniṣṭomasadbhavati tadvai tadyajamāna eva yajamānasya hyeṣa tadātmanaḥ

For, what part of him there is below the navel, that part of his self is this (Dhruva libation). Hence were he to slip away or discharge urine before the offering of this (libation), he would discharge the Dhruva (the firm, constant one): hence, lest he should discharge the Dhruva, he sits through the praise of Agni. This, indeed, applies only to the sacrificer, for this (libation) is part of the sacrificer's self.