In the Brihadaranyaka Upanishad 1.4.10, in the famous mantra where Aham Brahma Asmi is contained, there is a passage that denounces difference between the jiva and Ishwara: yo anyām devatām upāste…    

अथ योऽन्यां देवतामुपास्तेऽन्योऽसावन्योऽहमस्मीति न स वेद यथा पशुरेवं स देवानाम् । 

[if a man worships another deity, thinking: “He is one and I am another,” he does not know. He is like an animal to the gods.]

For this line Shankara’s commentary is as follows:

तद्यथेहैव तावत् — अथ यः कश्चिदब्रह्मवित् , अन्यामात्मनो व्यतिरिक्तां यां काञ्चिद्देवताम् , उपास्ते स्तुतिनमस्कारयागबल्युपहारप्रणिधानध्यानादिना उप आस्ते तस्या गुणभावमुपगम्य आस्ते — अन्योऽसावनात्मा मत्तः पृथक् , अन्योऽहमस्म्यधिकृतः, मया अस्मै ऋणिवत्प्रतिकर्तव्यम् — इत्येवंप्रत्ययः सन्नुपास्ते, न स इत्थंप्रत्ययः वेद विजानाति तत्त्वम् ।

Translation (Swami Madhavananda):  

As, for instance, here. While he, one who is not a knower of Brahman, who worships any god,  a god different from himself, approaches him in a subordinate position, offering him praises, salutations, sacrifices, presents, devotion, meditation, etc., thinking, ‘He is one, non-self, different from me, and I am another, qualified for rites, and I must serve him like a debtor’- worships him with such ideas, does not know the truth. He, this ignorant man, has not only the evil of ignorance, but is also like an animal to the gods.

Om