《礼记·文王世子》
古《记》上说： 虞夏商周四代，有师、保、疑、承作为太子的辅佐。
《记》曰： 虞、夏、商、周，有师保，有疑丞。 
The ancient Records state that the four dynasties of Yu, Xia, Shang and Zhou appointed teachers (shi), guardians (bao), doubters (yi) and supporters (cheng) to assist the prince.