On Osiris and Isis

"For it is a fact that most people do not understand that most general, and insignificant circumstance, for what reason the priests cut off their hair, and wear linen robes: some do not trouble themselves at all to know the cause for these two rules, whilst others say that they abstain from the use of wool, as they do from the flesh, out of veneration for the sheep; that they shave the head in token of their mourning (for Osiris), and that they wear linen on account of the colour the flax in blossom displays, which resembles the smiling atmosphere encompassing the earth."

Plutarch, Morals, "On Osiris and Isis, Chapter Four"