Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.1.7

7.5.1.7

yadveva kūrmamupadadhāti | prāṇo vai kūrmaḥ prāṇo hīmāḥ sarvāḥ prajāḥ karoti prāṇamevaitadupadadhāti tam purastātpratyañcamupadadhāti purastāttatpratyañcam prāṇaṃ dadhāti tasmātpurastātpratyaṅ prāṇo dhīyate puruṣamabhyāvṛttaṃ yajamāne tatprāṇaṃ dadhāti dakṣiṇato 'ṣāḍhāyai prāṇo vai kūrmo vāgaṣāḍhā prāṇo vai vāco vṛṣā prāṇo mithunam

And, again, why he puts down a tortoise;--the tortoise (kûrma) is the breath, for the breath makes (kar) all these creatures: it is breath he thus puts into him (Agni). He puts it down in front looking towards the back: he thus puts in the breath in front tending towards the back; whence the breath is taken in from the front backwards. [He puts it down so as to be] turned towards the (gold) man: he thus puts breath into the Sacrificer. South of the Ashâdhâ (he puts it), for the tortoise is breath, and the Ashâdhâ speech; and the breath (prâna, masc.) is the male, the mate, of speech (vâk, fem.).