The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
4. That (viz. breath) (is merged) in the ruler (i.e. the individual soul), on account of the (statements as to the prânas) coming to it and so on.
We have ascertained that a thing which has not originated from another is not itself merged in the latter, but only through its functions. A doubt now arises whether, according to the word of scripture, the function of breath is merged in heat, or in the individual soul which is the ruler of the body and senses.--According to the pûrvapakshin we must conclude that the breath is merged in heat only, since the scriptural statement allows no room for doubt and we are not entitled to assume something not declared by scripture. The breath under discussion persists 'in the ruler,' i.e. the intelligent Self (the individual soul) which possesses nescience, work, and former knowledge as limiting adjuncts; i.e. the function of breath has that soul for its substratum.--Why so?--'On account of (the prânas) going towards him,' &c.--Another scriptural passage declares that all prânas without any difference go to the soul, 'All the prânas go to the Self at the time of death when a man is thus going to expire' (Bri. Up. IV, 3, 38). Another passage again specially declares that the prâna with its five functions follows the individual soul, 'After him thus departing the prâna departs, 'and that the other prânas follow that prâna, 'And after the prâna thus departing all the other prânas depart' (Bri. Up. IV, 4, 2). And the text, 'He is furnished with intelligence' (ibid.), by declaring the individual soul to
be of intimately intelligent nature, suggests that in it, viz. the soul, the prâna--into which the different organs of knowledge have been merged--has taken its abode.--But scripture also says, 'The prâna (is merged) in heat;' why then make the addition implied in the doctrine--that breath is merged in the individual soul?--We must make that addition, we reply, because in the process of departure &c. the soul is the chief agent, and because we must pay regard to specifications contained in other scriptural passages also.--How then do you explain the statement, 'Breath is merged in heat?'--To this question the next Sûtra replies.