Shatapatha Brahmana 1.1.1.21

1.1.1.21

tā nāntareṇa saṃcareyuḥ | nenmithunaṃ caryamāṇamantareṇa saṃcarāniti tā nātihṛtya sādayenno anāptāḥ sādayetsa yadatihṛtya sādayedasti vā agneścāpāṃ ca vibhrātṛvyamiva sa yatheva ha tadagnerbhavati yatrāsyāpa upaspṛśantyagnau hādhi bhrātṛvyaṃ vardhayedyadatihṛtya sādayedyadya anāptāḥ sādayenno hābhistaṃ kāmamabhyāpayedyasmai kāmāya praṇīyante tasmādu sampratyevottareṇāhavanīyam praṇayati

Let nobody pass between the water (and the fire), lest by passing between them he should disturb the copulation which is taking place. Let him set the water down without carrying it beyond (the north side of the fire, i.e. not on the eastern side); nor should he put it down before reaching (the north side, i.e. not on the western side). For, if he were to put the water down after carrying it beyond,--there being, as it were, a great rivalry between fire and water,--he would cause this rivalry to break forth on the part of the fire; and when they (the priests and the sacrificer) touch the water of this (vessel), he would, by carrying it and setting it down beyond (the northern side), cause the enemy to rise (spirt) in the fire. If, on the other hand, he were to put it down before gaining (the northern side), he would not gain by it the fulfilment of the wish for which it has been brought forward. Let him therefore put it down exactly north of the Âhavanîya fire.