Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.1.8

3.7.1.8

atha yūpaśakalam prāsyati | tejo ha vā etadvanaspatīnāṃ yadbāhyāśakalastasmādyadā bāhyāśakalamapatakṣṇuvantyatha śuṣyanti tejo hyeṣāmetattadyadyūpaśakalam prāsyati satejasam minavānīti tadyadeṣa eva bhavati nānya eṣa hi yajuṣkṛto medhyastasmādyūpaśakalam prāsyati

He then throws in the (first) chip of the stake. Now that chip of the outer (bark) doubtless is the vigour of trees; hence, when a chip of their outer (bark) is cut off they dry up, for it is their vigour. Hence, when he throws in the chip of the stake, he does so thinking, 'I will plant it (the stake) full of vigour.' The reason why it is this (chip) and no other, is that this one has been produced with a formula, is sacrificially pure: therefore he throws in the chip of the stake.