I need to Snap a 3d object to a grid when it is placed in game.

Hello Unity answers community. I have looked all over the pages and could not find a script for snapping a 3d object to a grid when placed in the run time. I have a script hat is using a ray-cast to get the position of the courser that is locked at the center of the screen.

 var building : GameObject;
  var gridX = 5;
  var gridY = 5;
  var spacing = 2.0;
  function Update () 
  {
  
      if(Input.GetMouseButtonDown(0))
      {
          Build();
      }
  }
  function Build()
  {
          var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
          var hit : RaycastHit;
          if (Physics.Raycast (ray, hit, 100)) 
          {
              Debug.DrawLine (ray.origin, hit.point);
              Instantiate(building,hit.point,Quaternion.identity);
          }
  }

I need the Object (Cube) to be snapped to a grid when the object is spawned in.