《周书·本纪·卷五》
三年的守丧期，上至于天子也要遵守，这是古今不变的法则，是帝王经常实行的。
三年之丧，达于天子，古今无易之道，王者之所常行。
The three-year mourning period, up to the emperor, is an unchanging rule of ancient and modern times, and it is something that emperors have always done.