Bamidbar Rabbah
An idolater asked Rabban Gamliel, "Why was the Holy One, Blessed be He revealed to Moses in the Burning Bush?" Rabban Gamliel replied to him, "If God had been revealed in a carob tree or a fig tree, you would have asked me the same thing, and I could not send you away without an answer. This teaches you that there is no place in the world devoid of the Shekhinah."