bool isgrounded = true;

 void Update()
 {
     if (isgrounded == true)
     {
         //Do your action Here...
     }
 }

 //make sure u replace "floor" with your gameobject name.on which player is standing
 void OnCollisionEnter(Collision theCollision)
 {
     if (theCollision.gameObject.name == "floor")
     {
         isgrounded = true;
     }
 }

 //consider when character is jumping .. it will exit collision.
 void OnCollisionExit(Collision theCollision)
 {
     if (theCollision.gameObject.name == "floor")
     {
         isgrounded = false;
     }
 }