SB 11.13.23

pañcātmakeṣu bhūteṣu
 samāneṣu ca vastutaḥ
ko bhavān iti vaḥ praśno
 vācārambho hy anarthakaḥ
Word for word: 
pañca — of five elements; ātmakeṣu — made of; bhūteṣu — thus existing; samāneṣu — being the same; ca — also; vastutaḥ — in essence; kaḥ — who; bhavān — are You; iti — thus; vaḥ — your; praśnaḥ — question; vācā — merely with words; ārambhaḥ — such an endeavor; hi — certainly; anarthakaḥ — without real meaning or purpose.
Translation: 
If by asking Me “Who are You?” you were referring to the material body, then I must point out that all material bodies are constituted of five elements, namely earth, water, fire, air and ether. Thus, you should have asked, “Who are you five?” If you consider that all material bodies are ultimately one, being constituted essentially of the same elements, then your question is still meaningless, since there would be no deep purpose in distinguishing one body from another. Thus, it appears that in asking My identity, you are merely speaking words, without any real meaning or purpose.
Purport: 

Śrīla Viśvanātha Cakravartī Ṭhākura explains this verse as follows. “In the previous verse Lord Kṛṣṇa demonstrated that if the sages accepted the impersonal philosophy that all living beings are ultimately one in all respects, their question ‘Who are You?’ was meaningless, since there would be no philosophical basis to distinguish one manifestation of spirit soul from another. In this verse the Lord refutes the false identification with the material body composed of five elements. If the sages accepted the body as the self, then their question was meaningless, since they would have to ask, ‘Who are you five?’ If the sages replied that although the body is composed basically of five elements and these elements can be considered a single substance in the form of the whole body, then the Lord has already replied by the words samāneṣu ca vastutaḥ. The bodies of human beings, demigods, animals, etc., are all composed of the same five elements and are essentially the same. Therefore the question ‘Who are You?’ is ultimately meaningless. Thus, if one accepts either the theory that all living entities are ultimately the same or the theory that all living entities are ultimately nondifferent from their material bodies, in both cases the question of the sages is meaningless.

“The sages might argue that even among learned persons it is common practice to ask questions and give answers on many subjects as a part of normal life. The sages could point out that Lord Kṛṣṇa also distinguished among them, by His saying viprāḥ, ‘O brāhmaṇas,’ and vaḥ, or ‘your [question],’ as expressed in this verse. In this way it is seen that the Lord also accepts the ordinary customs of questions and answers. To answer this argument, Lord Kṛṣṇa states, vācārambho hy anarthakaḥ. The Lord states, ‘My addressing you as brāhmaṇas is merely an exhibition of words if we are ultimately not different. I merely reciprocated with your approach to Me. Therefore, if we are ultimately one, neither My statement nor your question has any real meaning. I can conclude therefore by your question to Me that you are all not actually very intelligent. Therefore, why are you inquiring after ultimate knowledge? Aren’t you all embarrassed?’”

Śrīla Madhvācārya points out in this regard that the question of the sages was not appropriate, since they had already seen their father, Lord Brahmā, worshiping the lotus feet of Lord Haṁsa. Since their spiritual master and father was worshiping Lord Haṁsa, they should have immediately understood the Lord’s position, and their question is thus meaningless.