《史记·八书·封禅书》
今天子自封禅始，其后十二年间，五岳、四渎遍祭一周。
今上封禅，其後十二岁而还，遍於五岳、四渎矣。
From the time of the Emperor's first Fengshan sacrifice, over a period of twelve years, he sacrificed at all five sacred mountains and the four great rivers.