Player slides down slope.

This question has been asked before but im having a hard time getting it to work for me. I have a 2D platformer where the Player has a Rigidbody 2D attached to it. As a lot of people know, if the player stands on a hill, the gravity (or something) from the Rigidbody will make the player slowly slide down the hill. How do I stop this from happening? Can I adjust something for the Rigidbody or even the slanted platforms that he is standing on? Here is some of the code I use for moving the player, any help is greatly appreciated!

 using UnityEngine;
 using System.Collections;
 
 
 public class Controls : MonoBehaviour
 {
     public Rigidbody2D rb;
     public float movespeed;
     public float jumpheight;
     public bool moveright;
     public bool moveleft;
     public bool jump;
     public Transform groundCheck;
     public float groundCheckRadius;
     public LayerMask whatIsGround;
     private bool onGround;
 
     // Use this for initialization
     void Start()
     {
         rb = GetComponent<Rigidbody2D>();
 
     }
 
     void FixedUpdate()
     {
         onGround = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround);
     }
 
     // Update is called once per frame
     void Update()
     {
 
 
 
         if (Input.GetKey(KeyCode.LeftArrow))
         {
             rb.velocity = new Vector2(-movespeed, rb.velocity.y);
 
         }
         if (Input.GetKey(KeyCode.RightArrow))
         {
             rb.velocity = new Vector2(movespeed, rb.velocity.y);
 
         }
 
         if (Input.GetKey(KeyCode.Space))
         {
             if (onGround)
             {
                 rb.velocity = new Vector2(rb.velocity.x, jumpheight);
             }
         }
 
         if (jump)
         {
             if (onGround)
             {
                 rb.velocity = new Vector2(rb.velocity.x, jumpheight);
             }
             jump = false;
         }
 
         if (moveright)
         {
             rb.velocity = new Vector2(movespeed, rb.velocity.y);
         }
         if (moveleft)
         {
             rb.velocity = new Vector2(-movespeed, rb.velocity.y);
         }
 
     }
 
 }