Shatapatha Brahmana 6.7.1.20

6.7.1.20

athādhyātmam | ātmaivāgniḥ prāṇāḥ śikyam prāṇairhyayamātmā śaknoti sthatuṃ yacaknoti tasmācikyam prāṇairevainametadbibharti ṣaḍudyāmam bhavati ṣaḍḍhi prāṇāḥ

Now as to the self (or body of Agni). Agni doubtless is the self, and the netting is the vital airs, for by means of the vital airs that self is able to exist; and inasmuch as it is so able (sak) therefore (the netting is called) 'sikya:' he thus carries (sustains) him by means of the vital airs; and it is furnished with six strings, because there are six vital airs.