Shatapatha Brahmana 6.6.1.23

6.6.1.23

muñjakulāyenāvastīrṇā bhavati | ādīpyāditi nveva yadveva muñjakulāyena yonireṣāgneryanmuñjo na vai yonirgarbhaṃ hinastyahiṃsāyai yonirvai jāyamāno jāyate yonerjāyamāno jāyātā iti

It is covered with a layer of Muñga grass, just for the purpose that it may blaze up. And as to why it is with a layer of Muñga grass, (it is done) to avoid injury, for that Muñga grass is a womb, and the womb does not injure the child; for he who is born, is born from a womb: 'May he (Agni), when he is born, be born from the womb,' thus he thinks.