《资治通鉴·唐纪·唐纪十三》
前宜州刺史郑无已经退休在家，太宗因为他过去曾跟从隋炀帝讨伐高丽，特意将他召到行宫，问他讨伐高丽的计策，郑元答道： 辽东路途遥远，运粮较为艰难。高丽人善于守城，攻城不能很快攻下。 
前宜州刺史郑元璹，已致仕，上以其尝从隋炀帝伐高丽，召诣行在；问之，对曰： 辽东道远，粮运艰阻；东夷善守城，攻之不可猝下。 
The former Prefect of Yizhou, Zheng Yuan, had already retired and returned home, but because he had accompanied Emperor Yang of Sui in his campaign against Goguryeo in the past, Emperor Taizong specially summoned him to his temporary palace and asked him about strategies for attacking Goguryeo, Zheng Yuan replied: The distance to Liaodong is great, so it will be difficult to transport grain. The people of Goguryeo are skilled in defending their cities, so it will not be easy to attack them and take their cities.