<a href="#" class="cp_btn">button</a>

a.cp_btn {
  position: relative;
  display: block;
  width: 160px;
  padding: 0.8em;
  text-align: center;
  text-decoration: none;
  color: #fff;
  background: #2196F3;
  border:solid 1px #fff;
  overflow: hidden;
}
a.cp_btn:before {
  position: absolute;
  top: -25px;
  left: -20px;
  content:'';
  height:90px;
  width:20px;
  background : #fff;
  opacity: 0.3;
  transform: rotate(40deg);
  transition: 0.4s ease-in-out;
}
a.cp_btn:hover:before {
  left:98%;
}