Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.2.18

3.5.2.18

atha yadvṛṣṇe stukā bhavati | vṛṣṇerha vai viṣāṇe antareṇāgnirekāṃ rātrimuvāsa tadyadevātrāgnernyaktaṃ tadihāpyasaditi tasmādvṛṣṇe stukā bhavati tasmādyā śīrṣṇo nediṣṭhaṃ syāttāmācidyāharedyadyu tāṃ na vindedapi yāmeva kāṃ cāharettadyatparidhayo bhavanti guptyā eva dūra iva hyenamuttare paridhaya āgacanti

And as to why there is a wether's hair-tuft,--Agni, forsooth, dwelt once for one night between the two horns of a wether: 'Whatever of Agni's nature is inherent therein, let that be here too,' so he thinks, and therefore there is a wether's hair-tuft. Let him, therefore, cut off that (tuft) which is nearest to the head, and bring it; and if he be unable to procure that, let him bring any kind (of wether's hair). And why there are enclosing-sticks?--for the protection (of Agni): for it is somewhat long before the next enclosing-sticks will come nigh to him.