prophet Muhammad (salallahu alayhiwasalam) said: "whoever attaches or wears a talismen(amulet) has commited shirk" (ahmed, tirmidi) "amulets, charms and all forms of bewitchment are acts of shirk"(ahmed, abu dawood) the reason its shirk is one is associating partners with Allah with regards to His names and attributes. we all know that Allah alone protects, provides and sustains His creation. if we say for example this horse shoe, this tree or four leaf clover can neither ward off, harm or bring benefit, its Allah alone who protects and Allah alone who blesses with His bounty.