《淮南子·齐俗训》
武王讨伐纣王的时候，用车载着去世不久的父亲的灵柩前去讨伐，等杀死纣王后，海内还没有安定下来，所以武王为文王守三年孝，以表示发扬文王的美德，这样才有了服三年之丧的做法。
武王伐纣，载尸而行，海内未定，故不为三年之丧始。
When King Wu fought King Zhou, he used a carriage to carry the coffin of his recently deceased father. After killing King Zhou, the country was not yet pacified, so King Wu mourned King Wen for three years to uphold King Wen's virtues, which is how the practice of three-year mourning came about.