In his commentary on Adhyaya 4 Pada 1 Sutra 3 of the Brahma Sutras, Adi Shankaracharya presents a number of scriptural quotes to support his Advaita view that the individual soul of jivatma is the same as Brahman. In particular, he says this:
The highest Lord must be understood as the Self. For in a chapter treating of the highest Lord the Gâbâlas acknowledge him to be the Self, 'Thou indeed I am. O holy divinity; I indeed thou art, O divinity!'
My question is, what is the text of "the Jabalas" that is quoted here? I assume the Jabalas were a Vedic Shakha or school related the sage Jabili, who tried to use atheist arguments to convince Rama to come back to Ayodhya, as described in the Ayodhya Kanda of the Ramayana. (See my answer here for more information on Vedic Shakhas.)
Now there is an Upanishad called the Jabala Upsnishad, which you can read here, and a Shaivite Upanishad called the Brihadjabala Upanishad, which you can read here, but neither one of them contain the verse I'm looking for.
I did find the verse in a Vaishnava Upanishad called the Varaha Upanishad, which you can read here:
Prostrations—prostrations to me who am in all the elements, who am the Chiḍāṭmā (viz., Āṭmā of the nature of wisdom) that is eternal and free and who am the Praṭyagāṭmā. O Ḍevaṭā, you are I. I am you. Prostrations on account of myself and yourself who are infinite and who are Chiḍāṭmā, myself being the supreme Īśa (Lord) and yourself being Śiva (of a beneficent nature).
But it's unlikely this is the text that Adi Shankaracharya was quoting from, since it has nothing to with the Jabalas. The Varaha Upanishad may be quoting some older text here.
I should add that this verse is also quoted in Adi Shankacharya's commentary on the Vishnu Sahasranamam, Vidyaranya's work the Brahmavid-Ashirvada, and Ramanujacharya's commentary on the Brahma Sutras (see here). If it helps, here is the verse in Sanskrit:
tvaṁ vā ahamasmi bhagavō dēvatē'haṁ vai tvamasi
