The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
27. And because Brahman is called the source.
Brahman is the material cause for that reason also that it is spoken of in the sacred texts as the source (yoni); compare, for instance, 'The maker, the Lord, the person who has his source in Brahman' (Mu. Up. III, i, 3); and 'That which the wise regard as the source of all beings' (Mu. Up. I, 1, 6). For that the word 'source' denotes the material cause is well known from the use of ordinary language; the earth, for instance, is called the yoni of trees and herbs. In some places indeed the word yoni means not source, but merely place; so, for instance, in the mantra, 'A yoni, O Indra, was made for you to sit down upon' (Rik. Samh. I, 104, 1). But that in the passage quoted it means 'source' follows from a complementary passage, 'As the spider sends forth and draws in its threads,' &c.--It is thus proved that Brahman is the material cause of the world.--Of the objection, finally, that in ordinary life the activity of operative causal agents only, such as potters and the like, is preceded by reflection, we dispose by the remark that, as the matter in hand is not one which can be known through inferential reasoning, ordinary experience cannot be used to settle it. For the knowledge of that matter we rather depend on Scripture altogether, and hence Scripture only has to be appealed to. And that Scripture teaches that the Lord who reflects before creation is at the same time the material cause, we have already explained. The subject will, moreover, be discussed more fully later on.