This paper presents a proximity-based authentication approach for the Internet of Things (IoT) that works in-band by solely utilizing the wireless communication interface. The novelty of this approach lies in its reliance on ambient radio signals to infer proximity within about one second, and in its ability to expose imposters located several meters away. We identify relevant features sensed from the RF channel to establish a notion of proximity across co-located low-power devices. We introduce our proximity-based authentication protocol and show the feasibility of our approach with an early prototype using off-the-shelf 802.15.4 sensors and an evaluation conducted in a real-world environment.