《资治通鉴·唐纪·唐纪十二》
太宗说： 隋炀帝疲劳百姓，修筑长城以防备突厥的进攻，最后毫无用处。
上曰： 隋炀帝劳百姓，筑长城以备突厥，卒无所益。
Emperor Taizong said: The Sui Dynasty Emperor Yang was exhausting the people by building the Great Wall to defend against the invasion of the Turks, but in the end it was useless.