Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.1.32

6.3.1.32

sā kalmāṣī syāt | sā hyāgneyī yadi kalmāṣīṃ na vindedapyakalmāṣī syātsuṣirā tu syātsaivāgneyī saiṣā yoniragneryadveṇuragniriyam mṛnna vai yonirgarbhaṃ hinastyahiṃsāyai yonervai jāyamāno jāyate yonerjāyamāno jāyātā iti

It (the spade) should be spotted, for such a one is of Agni's nature. If he cannot procure a spotted one, it may be unspotted, but hollow it must be, to guard (Agni) from injury;--(for) such a one alone is of Agni's nature; that, the bamboo, is Agni's womb; and this (lump of) clay is Agni; and the womb does not injure the child. For it is from a womb that he who is born is born: 'from the womb he (Agni) shall be born when he is born,' so he thinks.