Brahma Sutra 1.2.26

If it be said that (Vaisvanara) is not (Brahman) because of the word (‘Vaisvanara’, which has a definite meaning, viz. gastric fire) and other reasons, and on account of its existing inside (which is true of gastric fire), (we say) not so, because there is the instruction to conceive (Brahman) as such (as the gastric fire), because it is impossible (for the gastric fire to have the heaven etc. for its head and other limbs) and also because (the Vajasaneyins) describe him (Vaisvanara) as a person (which the gastric fire is not).