The purpose of this study was to investigate what kinds of sources people prefer to use when they answer questions online, especially, in the context of social Q&A. Social Q&A is a Web-based service, that allows people to ask questions and receive answers from their fellow users. In social Q&A, people often cite sources of information when they answer questions. It could be a name, a short description, or hyperlinks to the original sources. Yahoo! Answers was chosen for this study due to its popularity as a top ranked social Q&A service as well as its capability for separately indicating sources for the answers in its format. We collected data with a crawler that used Yahoo! Answers APIs. A total number of 5,391 sources were identified and analyzed with the following three approaches: (1) source distribution by online accessibility, (2) source distribution by genre, and (3) source distribution by subjects. At the early stage of this study, it was expected that the results of source preferences heavily relied on sources online, since people ask and answer questions on the Web-based service. Nevertheless, it was found that human (56.4%) was the most frequently cited type of source, and it was followed by online (40%) and offline sources (4%). According to the source distribution by genre, human (56.4%) was followed by the Internet (38.1%), books (3.6%), and mass media (1.6%), and the sub-categories of these sources were analyzed. Additionally, the patterns of source distribution were shown differently across subjects.