The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
45. Moreover it is so stated in Smriti.
In the Îsvaragitâs (Bhagavad-gîtâ) also it is said that the soul is a part of the Lord, 'an eternal part of me becomes the individual soul in the world of life' (Bha. Gî. XV, 7). With regard to the assertion made above, viz. that in ordinary life the relation of ruler and ruled is known to hold good in the case of master and servant &c. only, we remark that, although that may be the case in ordinary life, we ascertain from scripture that the relation of part and whole and that of ruler and ruled may go together. Nor is there anything contradictory in assuming that the Lord who is provided with superexcellent limiting adjuncts rules the souls which are connected with inferior adjuncts only.
Here the pûrvapakshin raises another objection. If we admit that the souls are parts of the Lord, it follows that the Lord also, whose part the soul is, will be afflicted by the pain caused to the soul by its experience of the samsâra-state; as we see in ordinary life that the entire Devadatta suffers from the pain affecting his hand or foot or some other limb. Herefrom it would follow that they who obtain Brahman obtain a greater pain 1: so that the former samsâra-condition would be preferable, and complete knowledge be devoid of purpose.--To this the following Sûtra replies.
63:1 Viz. by participating in all pain.