The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
REVERENCE TO THE HIGHEST SELF!
1. Speech (is merged) in mind, on account of this being seen, and of the scriptural statement.
Being about to describe the path of the gods which leads those who possess the lower kind of knowledge towards the attainment of their reward, the Sûtrakâra begins by explaining, on the basis of scriptural statements, the successive steps by which the soul passes out of the body; for, as will be stated later on, the departure of the soul is the same in the case of him who possesses the (lower) knowledge and of him who is devoid of all knowledge.
About the process of dying we have the following passage, 'When a man departs from hence his speech merges in his mind, his mind in his breath, his breath in fire, fire in the highest deity' (Kh. Up. VI, 6, 1). A doubt here arises whether the passage means to say that speech itself, together with its function, is merged in the mind, or only the function of speech.
The pûrvapakshin maintains that speech itself is merged in the mind. For this explanation only is in agreement with the direct statement of the sacred text, while the other alternative compels us to have recourse to an implied meaning; now wherever direct enunciation and implied meaning are in conflict the preference has to be given to the former, and we therefore maintain that speech itself is merged in the mind.
To this we reply that only the function of speech is merged in the mind.--But how can this interpretation be maintained, considering that the teacher (in the Sûtra) expressly says 'Speech in the mind?'--True we reply; but later on he says 'There is non-division, according to scriptural statement' (Sûtra 16), and we therefrom conclude that what is meant in the present Sûtra is merely cessation of the function of speech. For if the intention were to
express absorption of the thing (i.e. the organ of speech) itself, there would be 'non-division' in all cases, and for what reason then should 'non-division' be specially stated in another case (i.e. in the case of which Sûtra 16 treats)? The meaning therefore is that the different functions are retracted, and that while the function of the mind continues to go on the function of speech is retracted first.--Why so?--'Because this is seen.' It is a matter of observation that while the mind continues to act the function of speech comes to an end; nobody, on the other hand, is able to see that the organ of speech itself, together with its function, is merged in the mind.--But are we not justified in assuming such a merging of speech in the mind, on the ground of scriptural statement?--This is impossible, we reply, since mind is not the causal substance of speech. We are entitled to assume only that a thing is merged in what is its causal substance; a pot e.g. (when destroyed) is merged in clay. But there is no proof whatever for speech originating from mind. On the other hand we observe that functions originate and are retracted even where they do not inhere in causal substances. The function of fire, e.g. which is of the nature of heat, springs from fuel which is of the nature of earth, and it is extinguished in water.--But how do you, on this interpretation, account for the scriptural statement that 'speech is merged in the mind?'--'And on account of the scriptural statement,' the Sûtrakâra replies. The scriptural statement also may be reconciled with our interpretation, in so far as the function and the thing to which the function belongs are viewed as non-different.