The title of "khagan" for a leader of some groups of Rus' people is mentioned in several historical sources, most of them foreign texts dating from the 9th century, while three East Slavic sources date from the 11th and 12th centuries.
Ahmad ibn Rustah, a 10th-century Muslim geographer from Persia, wrote that the Rus' khagan ("khāqān rus") lived on an island in a lake [REF].