It says here that Shri Adi Shankaracharya says this in his Brahma Sutra Bhashya.
Now this Saguna Brahman is he with a particular form as per Shankara, let us look at what he says “मन्त्रौ चेमावीश्वरस्य शरीराद्यनपेक्षतामनावरणज्ञानतां च दर्शयतः न तस्य कार्यं करणं च विद्यते न तत्समश्चाभ्यधिकश्च दृश्यते। परास्य शक्तिर्विविधैव श्रूयते स्वाभाविकी ज्ञानबलक्रिया च इति। अपाणिपादो जवनो ग्रहीता पश्यत्यचक्षुः स श्रृणोत्यकर्णः। स वेत्ति वेद्यं न च तस्यास्ति वेत्ता तमाहुरग्र्यं पुरुषं महान्तम् इति च“(Brahma Sutra Chapter 1 Sutra 5)
Translation ” The Mantra here describes Ishwara who is without any need to body and whose knowledge is not covered by anything. “of him there is no cause nor effect, neither is there anyone equal to him nor greater. The power of that Supreme creates everything being of the nature of knowledge, power and activity. He is without feet or arms yet he can walk and grasp, he hears without ears. He knows everything that is to be known but none knows him. Such a person is called the great Purusha” so said by the Mantra”
But isn't Ishwara the nimitta karana or efficient cause of the universe, according to Advaita?