זוהר שמות — דף ריד ע”ב

On the side of holiness, letters are subtracted to increase sanctity, while on the other side letters are added, representing deficiency. The Ark of the Covenant belongs to the holy side and is destined to receive the form of the holy person. Pious sages who passed away were placed in a coffin because they had preserved the holy covenant. Joseph merited being placed in a coffin because he guarded the covenant throughout his life. One who violates the covenant and is placed in a coffin desecrates it and faces eternal retribution.