Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.11

3.8.3.11

yatra vai devāḥ | agre paśumālebhire taṃ tvaṣṭā śīrṣato 'gre 'bhyuvāmotaivaṃ cinnālabheranniti tvaṣṭurhi paśavaḥ sa eṣa śīrṣanmastiṣko 'nūkyaśca majjā tasmātsa vānta iva tvaṣṭā hyetamabhyavamattasmāttaṃ nāśnīyāttvaṣṭurhyetadabhivāntam

For when the gods, at first, seized an animal to sacrifice), Tvashtri first spat upon its head, thinking, 'Surely, thus they will not touch it!' for animals belong to Tvashtri. That (spittle became) the brain in the head and the marrow in the neck-bone: hence that (substance) is like spittle, for Tvashtri spat it. Let him therefore not eat that, since it was spitten by Tvashtri.