Locating the propagation source is one of the most important strategies to control the harmful diffusion process on complex networks. In this paper, we define two observers based similarity measures, including Infection Time Similarity and Infection Time Order Similarity. On the other hand, direction-induced search (DIS, developed in our previous work) is an effective graph traversal method for source locating. By combining the two observers based similarity measures with relaxed DIS, we propose a novel source locating method. Assuming that the diffusion process on complex networks follows discrete Susceptible-Infectious (SI) model, we verify the performance of this method on a series of synthetic and real networks. The experimental results show that the proposed method is better than other three state-of-the-art methods in locating the propagation source.PACS 89.20.Ff · 89.75.Da