《北史·列传·卷七》
太子还是年幼读书的时候，不应当在白天时间放下书本到内室去，这不是使他弱体平安，得到长寿的办法。 
太子尚以幼年涉学之日，不宜于正昼之时，舍书御内，又非所以安柔弱之体，固永年之命。 
The crown prince is still young and in the period of studying, and should not put down his books during the day to go inside. This is not the way to help him become strong and healthy and live a long life.