The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
44. And on account of the mantra.
A mantra also intimates the same view. 'Such is the greatness of it; greater than it is the Person. One foot of it are all beings, three feet of it are the Immortal in heaven' (Kh. Up. III, 12, 6). Here the word 'beings'
denotes all moving and non-moving things, among which the souls occupy the first place; in accordance with the use of the word in the following passage, 'Not giving pain to any being (bhûta) except at the tîrthas' (Kh. Up. VIII, 15). Herefrom also we conclude that the individual soul is a part of the Lord.--And again from the following reason.