Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.18

1.1.1.18

tā utsicyottareṇa gārhapatyaṃ sādayati | yoṣā vā āpo vṛṣāgnirgṛhā vai gārhapatyastadgṛheṣvevaitanmithunam prajananaṃ kriyate vajraṃ vā eṣa udyacati yo 'paḥ praṇayati yo vā apratiṣṭhito vajramudyacati nainaṃ śaknotyudyantuṃ saṃ hainaṃ śṛṇāti

After pouring out some of it (into the jug) he puts it down north of the Gârhapatya fire. For water (ap) is female and fire (agni) is male; and the Gârhapatya is a house: hence a copulation productive of offspring is thereby effected in this house. Now he who brings forward the water, takes up a thunderbolt; but when he takes up the thunderbolt, he cannot do so unless he is firmly placed; for otherwise it destroys him.