《宋书·列传·卷八十二》
三年为父母守孝，是古今通用的制度。因为悲哀是真诚发生的，所以制定外在的规矩，时间一久便成习惯，所以规定很长时间而习惯礼制。
又三年之丧，天下之达丧，以其哀并衷出，故制同外兴；日久均痛，故愈迟齐典。
The three-year mourning period for parents is a system prevalent in both ancient and modern times. Because the sadness is genuine, an external guideline is set. After a long time, it becomes a habit, so a long period of time is set to abide by the rites and customs.