The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
4. If it be said (that the prânas do not go) on account of the scriptural statement as to entering into Agni, &c., we deny this on account of the metaphorical nature (of those statements).
Well, the pûrvapaksha resumes, we deny that at the time when a new body is obtained the prânas go with the soul, because scripture speaks of their going to Agni, &c. For that at the time of death speech and the other prânas go to Agni and the other gods the following passage expressly declares: 'When the speech of the dead person
enters into the fire, breath into the air,' &c. (Bri. Up. III, 2. l3).--To this we reply that the objection is of no force on account of the metaphorical character of those statements. The entering of speech, &c., into Agni is metaphorical, because we observe no such entering in the case of the hairs of the head and body. For although the text says that 'the hairs of the body enter into the shrubs and the hairs of the head into the trees;' still we cannot understand this to mean that the hairs actually fly away from the body and enter into trees and shrubs. On the other hand, the soul could not go at all if we denied to it the limiting adjunct formed by the prânas, and without the latter it could not, in the new body, enter into the state of fruition. Besides, other passages distinctly declare that the prânas go with the soul.--From all this we conclude that the passage about speech, &c. entering into Agni, metaphorically expresses that Agni and the other divinities who act as guides of the prânas and cooperate with them stop their cooperation at the time of death.