The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
18. (The soul after having passed forth from the body) follows the rays.
There is the vidyâ of him within the heart, which begins, 'There is this city of Brahman and in it the palace, the small lotus, and in it that small ether' (Kh. Up. VIII, 1, 1). A subsequent section of that chapter--beginning with the words, 'Now these veins of the heart'--describes at length the connexion of the veins and the rays, and the text then continues, 'When he departs from this body, he departs upwards by those very rays,' and further on, 'By that
moving upwards he reaches immortality.' From this we understand that the soul passing out by the hundred and first vein follows the rays.--A doubt here arises as to whether the soul of him who dies by night as well as of him who dies by day follows the rays, or the soul of the latter only.--Since scripture mentions no difference, the Sûtra teaches that the souls follow the rays in both cases.