Leviticus 19:27

The corners of your heads; i.e. your temples: Ye shall not cut off the hair of your heads round about your temples. This the Gentiles did, either for the worship of the devils or idols, to whom young men used to consecrate their hair, being cut off from their heads, as Homer, Plutarch, and many others write; or in funerals or immoderate mournings, as appears from Isaiah 15:2 Jeremiah 48:37. And the like is to be thought concerning the beard or the hair in the corner, i.e. corners of the beard. The reason then of this prohibition is, because God would not have his people agree with idolaters, neither in their idolatries, nor in their excessive sorrowing, no, nor so much as in the appearances and outward significations or expressions thereof.