Sources about Slavs and their beliefs mainly come from the old Ruthenian chronicles and notes of German bishops. A lot of information contains Helmold's Chronica Sclavorum. There's also Chronicle of Thietmar, and the other chronicles, although they are not the only sources of information about Slavs and their beliefs. There's also for example from the diary of a Jewish trader of Slavs - Ibrahim Ibn Jacob, but he was not the only one - there are also notes and relations of Ibn Rosteh, one authors of "Arabic Sources for the History of Slavdom", here is a full list of the Authors :