Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.2.15

7.1.2.15

taṃ haike tricitaṃ cinvanti | trayo vā ime vāñcaḥ prāṇā iti na tathā kuryādati te recayantyekaviṃśasampadamatho anuṣṭupsampadamatho bṛhatīsampadaṃ ye tathā kurvantyekaṃ hyevaitadrūpaṃ yonireva prajātireva yadete 'vāñcaḥ prāṇā yaddhi mūtraṃ karoti yatpurīṣam praiva tajjāyate

Now some build it (the Gârhapatya) in three layers, saying, 'There are here three downward vital airs.' Let him not do so: they who do so do what is excessive,--one amounting to twenty-one, one amounting to the Anushtubh, and one amounting to the Brihatî; for this (altar) is of one single form--a womb. And as to those downward vital airs, they are indeed a bringing forth, for even the urine and faeces he voids are 'brought forth.'